Showing papers in "Ardea in 2023"

Distribution and Numbers of Ground-Foraging Birds between the Hyper-Arid Sahara and the Hyper-Humid Guinea Forests

Abstract: This paper quantifies the density and the total number of granivorous and insectivorous ground-foraging birds, whether Afro-Palearctic migrants or Afro-tropical residents, in the transition zone between the arid Sahara and the humid Guinea zone. Situated between 17°W and 42°E and between 7°N and 22°N, this is an area covering 10 million km2. The study took place during the northern winter, between 20 November and 10 March (thus covering much of the long dry season) from 2011 up to and including 2019. Using a stratified random sampling regime, we counted birds at 1901 sites of 4.5 ha in area. We present background information about the study region, with maps showing variation in elevation, rainfall, woody cover, land use and human population density. The bird counts were converted into average densities for 43 bird species in 150 grid cells of 1° latitude × 1° longitude. The distribution of the various bird species was predominantly related to annual rainfall, but because woody cover increases with rainfall, species' preferences for arid or more humid zones were partly influenced by an overall preference for open or more wooded landscapes. Bird species such as larks and Tawny Pipit Anthus campestris, even when rainfall was accounted for, selected comparatively open landscapes, whereas species feeding on the ground near trees or using them as perches (e.g. sparrows, finches, shrikes, Tree Pipit Anthus trivialis) preferred relatively more enclosed environments. To estimate total population size, the 150 grid cells were assembled into eleven rainfall categories (per 100 mm rainfall) and six longitudinal bands. To assess the reliability of these estimations, population sizes were calculated separately on the 1901 study sites split in two halves. The estimated population sizes were precise for migrants, especially for insectivores (7% deviation for the split-half estimates), but less precise for residents (22–28% deviation). Most ground-foraging birds were granivorous (at least in the dry season), their total number being estimated at 4000 million residents and 133 million migrants, residents being 30 times as abundant as migrants. Ground-foraging insectivores were less numerous, the total estimated being 920 million birds, of which 694 million were residents and 221 million migrants, the ratio residents/migrants being an order of magnitude smaller than in granivores. The three most abundant granivorous residents were Red-cheeked Cordon-bleu Uraeginthus bengalus (467 million), Sudan Golden Sparrow Passer luteus (375 million birds) and Red-billed Quelea Quelea quelea (311 million). The Greater Short-toed Lark Calandrella brachydactyla (126 million) was the only common granivorous migrant. The most common insectivorous ground-foraging bird was a resident (Greater Blue-eared Starling Lamprotornis chalybaeus; 100 million), and more commonly encountered than all the ground-foraging insectivorous migrants such as Isabelline Wheatear Oenanthe isabellina (32 million), Northern Wheatear Oenanthe oenanthe (27 million) and Western Yellow Wagtail Motacilla flava (24 million) together.

Distribution and Numbers of Arboreal Birds between the Hyper-Arid Sahara and the Hyper-Humid Guinea Forests

Abstract: This study quantifies the spatial variation in the density of 50 arboreal bird species (17 insectivorous Afro-Palearctic migrants and 33 Afro-tropical residents: 20 insectivores, 7 frugivores and 6 sunbirds) in the transition zone between the arid Sahara and the humid Guinea zone, measured during the dry season in 2011–2019. The distribution of bird species was related to annual rainfall, with Afro-Palearctic migrants found mainly in the relatively dry zone and Afro-tropical residents in the more humid zone. As woody cover increases with rainfall, bird species from the dry zone are by default found in more open habitats than species from the humid zone. This effect of woody cover largely dissolves when corrected for rainfall. The data – pertaining to absolute bird counts in stratified random sites – were used to estimate the total number of birds in this region. To assess the reliability of these estimations, population sizes were calculated separately per species on half-split data. The two estimates deviated about 10% from the averages calculated for the full data set. Among arboreal birds (1322 million), insectivorous residents were most abundant (547 million), followed by insectivorous migrants (326 million), nectarivorous residents (272 million) and frugivorous residents (177 million). The two most numerous arboreal bird species were insectivorous residents: Tawny-flanked Prinia Prinia subflava (128 million) and Green-backed Camaroptera Camaroptera brachyura (103 million). Among the migrants, the three most abundant were Subalpine Warbler Curruca iberiae + subalpina + cantillans (62 million), Lesser Whitethroat Curruca curruca (48 million) and Western Bonelli's Warbler Phylloscopus bonelli (30 million).

Seasonal Shifts in Habitat Choice of Birds in the Sahel and the Importance of ‘Refuge Trees’ for Surviving the Dry Season

Abstract: Every year, hundreds of millions of migratory birds cross the Sahara to spend the northern winter in the Sahel. After their arrival in September the region does not receive any rainfall until June while temperatures increase. Birds inhabiting the Sahel have several strategies to cope with this seasonal advent of drought. Most ground-foraging and arboreal migrants actually remain in the desiccating Sahel, although Northern Wheatear Oenanthe oenanthe remains in the arid zone only in a wet year, but moves from the arid to the semi-arid zone in a dry year. Some arboreal migrants stay for 1–2 months in the Sahel during the early dry season, but move on to the more humid zone further south for the rest of the northern winter. Common Redstart Phoenicurus phoenicurus is the only Sahelian arboreal migrant that moves southward in this period. Counter-intuitively, Curruca species move northward after the early dry season to the arid zone where they concentrate in woody plant species whose attractiveness increases later in the dry season. This is either because those plants then gain berries (Toothbrush Tree Salvadora persica) or because they develop flowers (six desert species). In the semi-arid zone, tree-dwelling bird species disappear from tree species when these lose their leaves. However, in tree species which do not shed their leaves, bird numbers remain either constant (those using Desert Date Balanites aegyptiaca) or increase (those using Winter Thorn Faidherbia albida, a tree that foliates during the dry season). On floodplains bird numbers in acacia trees increase during the dry season. As a consequence, birds become concentrated in fewer tree and shrub species during their stay in the Sahel. After wet rainy seasons, trees have more flowers and leaves and shed them later, giving the birds more foraging space. At the end of their stay in Africa after dry rainy seasons, the number of arboreal birds is only half that after wet rainy seasons, suggesting higher mortality in dry years. Clearly, in such years mortality would be even higher without what can be seen as ‘refuge trees’: the acacias on floodplains, and Faidherbia and to a lesser degree Balanites in the rest of the Sahel.

Selection by Birds of Shrub and Tree Species in the Sahel

Abstract: The Sahel is thinly covered by trees, but nevertheless forms an important habitat for millions of tree-dwelling birds. We describe tree availability and tree selection of 14 insectivorous Afro-Palearctic migrants and 18 Afro-tropical residents (10 insectivores, 3 frugivores and 5 nectarivores) inhabiting the Sahel from the Atlantic to the Red Sea. Of the 304 woody species identified across the region during systematic fieldwork in stratified plots, we noted height and canopy surface of 760,000 individual woody plants. Birds present in trees and shrubs were recorded separately per individual woody plant. 99.5% of the birds were concentrated in only 41 woody species. For 20 out of 32 bird species, Winter Thorn Faidherbia albida was the tree species most often used. Two other important tree species were Umbrella Thorn Acacia tortilis and Desert Date Balanites aegyptiaca. Representing only 11% of the total woody canopy cover, these three species attracted 89% of Western Bonelli's Warblers Phylloscopus bonelli and 77% of Subalpine Warblers Curruca iberiae + subalpina + cantillans. High selectivity for particular woody species was typical for migrants and residents, irrespective of their diet. Bird species feeding in shrubs used a larger variety of woody species than bird species feeding in tall trees. The highest bird densities (80–160 birds/ha canopy) were found in three shrubs with a limited distribution in the southern Sahara and northern Sahel: the berry-bearing Toothbrush Tree Salvadora persica, the small thorny shrub Sodad Capparis decidua and the small tree Maerua crassifolia. Other bird-rich woody species were without exception thorny (Balanites aegyptiaca, various species of acacia and ziziphus). In contrast, the five woody species most commonly distributed across the region (Cashew Anacardium occidentale, African Birch Anogeissus leiocarpus, Combretum glutinosum, Guiera senegalensis and Shea Tree Vitellaria paradoxa), representing 27% of the woody cover in the study sites, were rarely visited by foraging birds. In this sub-Saharan region, it is not total woody cover per se that matters to birds, but the presence of specific woody species. This finding has important implications: remote sensing studies showing global increase or decline of woody vegetation without identifying individual species have little value in explaining trends in arboreal bird populations. Local people have a large impact on the species composition of the woody vegetation in the Sahel, with positive and negative consequences for migrants wintering in this region. Faidherbia albida, the most important tree for birds in the sub-Saharan dry belt, is highly valued by local people and has the distinction of leafing in winter and being attractive to arthropods. On the other hand, migratory and African bird species have been negatively affected by the rapidly expanding cashew plantations since the early 1980s.

Downstream Ecological Consequences of Livestock Grazing in the Sahel: A Space-For-Time Analysis of the Relations between Livestock and Birds

Abstract: Bird counts in the Ferlo, NW Senegal had shown that the density of ground-foraging birds was much lower in grazed than in ungrazed savannah: 84% fewer granivorous birds and 64% fewer insectivorous birds. Between 1960 and 2010, in three areas within the same region, granivores declined by 39–97% and insectivores by 61–91%, losses attributable to the steadily increasing livestock grazing pressure. If these trends hold for all Sahelian rangelands, the extrapolation at the time indicated that 1.5 billion birds would have been lost in just half a century. The aim of this space-for-time substitution study was to investigate whether that extrapolation can be substantiated with data from the eastern Sahel. To permit analysis of the spatial and seasonal variation in grazing pressure and its impact on ground-foraging granivorous and insectivorous birds, we counted birds in 1901 sites across the entire region (Mauritania to Ethiopia) and took landscape photos of these sites to evaluate livestock presence. We also estimated livestock density from our counts of cowpats and of droppings of sheep and goats, and the cover of the soil vegetation. Within the same rainfall zone (200–400 mm/year), the grazing pressure was higher in the Ferlo than elsewhere in the Sahel. Grazing pressure declined in the Ferlo in the course of the dry season, indicating that cattle food supply became depleted. No such seasonal decline was recorded elsewhere in the Sahel. The same pattern was found for ground-foraging birds: a decline during the dry season in the Ferlo, but nowhere else in the same Sahel rainfall zone. Indeed, in the 1960s and 1970s, when the grazing pressure was much lower than today, there were no records of a seasonal decline of ground-foraging birds in the Ferlo. The much lower densities of seed-eating birds in the Ferlo were not exhibited elsewhere in the Sahel, which is consistent with the phenomenon of local overgrazing. The previous extrapolated loss of 1.5 billion birds is therefore too high, because conditions in the Ferlo were found to differ from those experienced elsewhere in Sahel's arid zone. The actual overall loss cannot be specified due to lack of bird counts from the past in the eastern Sahel. The comparison of grazing pressure and bird densities across all rainfall zones of the western and eastern Sahel shows that increasing livestock densities negatively impact bird numbers. Livestock grazing pressure in the west was higher than in the east and most ground-foraging bird species were less common in the west than in the east. Furthermore, the majority of ground-foraging species in West Sahelian savannahs were exclusively confined to the arid and semi-arid zone, but in the east, these species were more widely distributed and also occupied the more humid zone to the south.

The Gap of Chad, a Dearth of Migratory Birds in the Central Sahel

Abstract: Many migratory bird species cross the Mediterranean during autumn migration, but most do so either at the western or eastern ends where they can avoid, or minimise, sea crossings. The intervening 3500 km has long sea crossings, probably adding to the barrier imposed by the Sahara. If this were the general migration pattern, it would result in high concentrations of Afro-Palearctic migrants in West and East Africa and fewer in the central sub-Saharan zones. Unless migrants reorientate upon reaching the sub-Sahara, densities of migratory birds in the central Sahel should be much lower than at either end of the African savannah range. The available studies of birds equipped with GPS or geolocators show that south of the Sahara at least some species perform lateral movements to some extent. However, many remain either in the Sahel's western or eastern parts or continue moving southwards along the same longitudinal axis. We use density counts of arboreal birds from across the full width of the Sahel to explore the extent to which the central Sahel zone is underused by migratory birds. Eleven out of twelve common migratory arboreal species occurred at lower densities in the central Sahel than could be explained by tree-related variables. Western Bonelli's Warbler Phylloscopus bonelli, Western Orphean Warbler Curruca hortensis and Subalpine Warbler Curruca cantillans were most common in the western and (much) less common in the central Sahel, whereas Eastern Olivaceous Warbler Iduna pallida, Eastern Orphean Warbler Curruca crassirostris, Lesser Whitethroat Curruca curruca and Rüppell's Warbler Curruca ruppeli were most common in eastern, but less so in the central Sahel. Woodchat Shrike Lanius senator and Common Redstart Phoenicurus phoenicurus were more common in the western and eastern parts than in the central Sahel. No longitudinal variation was found for Common Whitethroat Curruca communis, which is consistent with the knowledge that many cross the Mediterranean waters upon encountering them. The conclusion is justified that the central Sahel is underused by migratory birds and by consequence, as far as these birds are concerned, not ‘saturated’. The question arises whether in the past, when the number of migratory birds was much greater than today, there might not have been a Gap of Chad.

Granivorous Birds in the Sahel: Is Seed Supply Limiting Bird Numbers?

Abstract: During the dry season four billion African and European granivorous birds in the Sahel consume, by grand average, 15 g seeds/ha/day, equivalent to an average annual consumption of 4.5 kg/ha. This represents only 4–15% of the estimated average total soil seed bank of some 30–100 kg/ha in the early dry season. Despite this apparent abundance of food, there are many reasons to presume that the number of seed-eating birds is limited by their food supply. First, the birds have to share the seed supply with rodents and insects that eat more seeds than all the birds combined. Second, granivorous birds are constrained by foraging time available to them. They avoid foraging during the midday heat and feeding time is mostly restricted to the early morning and late afternoon, totalling about 4 h per day. This forces them to achieve high intake rates and thus to select feeding sites where the available seeds can be handled quickly and/or are so abundant that the encounter rate is high. Third, only a proportion of the seeds lies on the surface where they are easy to find. Most grass seeds are tiny and even small birds need to eat thousands per day. Because they have so little time to look for food, they cannot afford to search for seeds hidden in the sand. Doves rapidly swallow seeds whole, but all smaller seedeaters have to separate the husk from the seed, a process that takes time too. Fourth, seed-eating birds in the Sahel discriminate between seeds. They ignore ‘empty seeds’ (husks) and also avoid feeding on common graminoids whose seeds have long awns (Aristida) or spines (Cenchrus) and which are time-consuming to process. Occasionally, granivorous birds may select seeds from forbs, but these, being low in digestibility, are not the preferred choice. Granivorous birds prefer the seeds of Panicum grass and other grass species with highly soluble carbohydrate fractions. Birds switch to marginal seed types at the end of the dry season, when the seed bank of the preferred species is depleted. Fifth, soil seed bank of preferred grass species is much reduced in dry years. Panicum and other preferred annual grasses are found mostly on riverine floodplains and in depressions that are prone to ephemeral flooding during the rainy season. Such sites attract many seed-eating birds, but the total surface area of floodplains is relatively small compared to the extensive drylands, on top of being very much smaller in dry years, circumstances that account for high mortality among seed-eating birds in drought years. The final argument for food-limitation is that the mounting grazing pressure of livestock over the last decades has severely reduced the annual soil seed bank and changed the plant community (preferred grass species replaced by non-preferred grasses and forbs). The combination of these factors caused a very large decline of seed-eating bird populations in the Sahel between the 1970s and 2010, including a handful of Eurasian species. The Sahel is still home to some four billion granivorous birds during the dry season, but just half a century ago the numbers must have been much higher.

Frequent Agonistic Interactions among Arboreal Birds in Savannahs But Not in Humid Forests of Africa

Abstract: Arboreal birds in the Sahel are highly selective in their tree choice. Most migrant, but also resident, birds are found in a few tree species and within those species often only in trees with abundant leaves and flowers. For this reason alone, preferred trees were expected to teem with birds. This was not the case. Most bird species were present in trees as singletons, even half of the Senegal Eremomela Eremomela pusilla – the most social species of all – were recorded as solitary birds. The probability that two different bird species were in the same tree was also very small, 2.8% on average. Mixed-group foraging flocks of arboreal birds, as so often reported for tropical forests, did not occur in the Sahel. Perhaps birds forage singly because they have no need to fear the raptors that are common in the forests further south. Some species, such as European Pied Flycatcher Ficedula hypoleuca and Common Redstart Phoenicurus phoenicurus, defend winter territories, but for other species individual home ranges show overlap. In the humid forests further south, and among resident species in the Sahel, few agonistic interactions were seen, but migratory birds were often agonistic with congeners and even more frequently with birds of other species. Larger bird species usually won agonistic interactions, but Western Olivaceous Warblers Iduna opaca chased off birds twice their own body size. Subalpine Warbler Curruca iberiae + subalpina + cantillans and other Curruca species, with the exception of Lesser Whitethroat Curruca curruca, were also intolerant. Western Bonelli's Warblers Phylloscopus bonelli, and during migration also Willow Warblers Phylloscopus trochilus, were most often on the receiving end of agonistic interactions. Far fewer agonistic interactions were recorded in the more humid regions to the south of the Sahel. This disparity may hinge on the higher intra- and interspecific encounter rate in the Sahel, where a greater fraction of trees are occupied by birds, than in the humid forests.

The Fortunes of Migratory Birds from Eurasia: Being on a Tightrope in the Sahel

Abstract: Many studies have shown that rainfall in the Sahel has a great influence on population trends of European bird species that spend the northern winter there. African bird species living in the Sahel, notably those that forage on the ground, have also shown significant declines, but independent of rainfall. This paper summarises the results of field data gathered in the entire Sahel and evaluates the many factors that play a role in the fortunes of birds. (1) Rainfall determines the extent of open water in the Sahel, and by default the fortunes of waterbirds. In recent decades the surface area of open water has increased because water tables have risen. (2) Rainfall south of the Sahel determines river discharge and therefore the surface of floodplains in the Sahel. Rainfall has a cumulative effect: discharges disproportionally decrease after a number of years with little rain, and vice versa. During the dry season (October–May), floodplains gradually dry out. In wet years, water – and hence food – is available for birds up to their departure, but in dry years birds become concentrated at the few remaining pools and so present an easy target for bird-trappers. Further desiccation leads to starvation. (3) After a year with heavy rainfall, seed is available in abundance, but a dry year results in a shift in the plant community and a low seed supply. Mortality among seedeaters increases under dry conditions. (4) In dry years, trees lose their leaves early on, forcing arboreal birds into a diminishing number of trees that retain leaves. In extremely dry years trees die on a massive scale and it takes many years before tree coverage is restored. When droughts occur in quick succession, as in 1972/73 and again in 1984/85, tree recovery is slow and populations of arboreal birds will continue to decline, or recover slowly or only partly (as for Eurasian Wryneck Jynx torquilla and Common Redstart Phoenicurus phoenicurus, whose numbers remain reduced by tenfold when compared to the 1950s, despite a slight recovery). Rainfall in the Sahel gradually recovered after 1990, as did the woody vegetation albeit with a delay, and many migratory bird species responded accordingly. Subalpine Warblers Curruca subalpina and Western Orphean Warblers Curruca hortensis have increased as much as threefold to fivefold since 1990. Southern European bird species, wintering in the arid parts of southern Sahara and Sahel, were hit the hardest during the Great Drought in 1969–1992, but also recovered the fastest, particularly strongly once rainfall had significantly recovered. Despite clear links between migratory birds and rainfall-related variables in their wintering areas, a migrant's world is more complicated than exclusively being constrained by rainfall. In the past century, the human population in sub-Saharan Africa has increased tenfold, with far-reaching consequences. (1) Cattle numbers boomed and grazing pressure increased greatly. Heavy grazing means lower grass seed production, especially of seeds that birds prefer. The steep decline of granivorous birds over the past several decades is therefore no surprise. (2) Every year, 2% of the savannah is converted into farmland. This is unfavourable for most, but not all, bird species. Farmers selectively favour particular tree species on their land (and remove the rest), by which the resulting tree species composition differs from the typical variety found in savannah. The shift in tree composition particularly disfavours birds wintering in both the more arid and more humid vegetation zones. Arboreal birds wintering in the intermediate zone have benefitted from the expansion of farming, because their preferred tree (White Thorn Faidherbia albida) is favoured by farmers and has become more abundant. (3) The expansion of agriculture is most obvious in the low-lying, slightly more humid and more fertile soils, much to the detriment of flooded forests, which being important refuges during droughts are particularly rich in birds. Most flooded forests have disappeared from the Sahel. (4) Millions of hectares of humid woody savannah have been converted into Cashew Anacardium occidentale plantations since 1980. Cashew plantations are almost completely devoid of birds and so constitute a significant loss of a once highly diverse wooded habitat. (5) The proliferation of Prosopis juliflora, an exotic tree species resembling an acacia, has locally and regionally displaced indigenous acacia species. Prosopis attracts few birds in comparison with acacias. (6) Irrigation and dam construction have decimated the size of floodplains in Senegal, Mali, Nigeria and Chad, with corresponding effects on many bird species, especially waterbirds, that concentrate in these areas. (7) Massive bird catches, such as those in the Inner Niger Delta after about 1990, did not occur in the past when cheap nylon nets (initially used for fishing) were not yet available. Furthermore, storage and transportation of trapped birds were impossible until ice and vehicles became available. (8) Large bird species have virtually disappeared from the Sahel due to human predation, especially in the densely populated western part. As a wintering area for most migratory Eurasian birds, the Sahel has become less attractive. Far fewer migratory birds now use the Sahel than half a century ago, not least because many populations have substantially declined in numbers. Furthermore, several European migrant species increasingly are wintering north of the Sahara, which may relate to impoverished conditions in the Sahel, but as likely to significant habitat changes in Europe and the pace of climate change.

Revisiting Published Distribution Maps and Estimates of Population Size of Landbirds Breeding in Eurasia and Wintering in Africa

Abstract: For 30 Afro-Palearctic bird species, the size of the breeding population in Europe is compared to the numbers wintering in the northern dry belt of Africa south of the Sahara, the Sahel. As the distribution of most of these species is wider than just Europe and the Sahel, the estimates are adjusted based on known breeding and wintering ranges. Eight Palearctic species recorded sparsely in the Sahel appeared to winter mainly beyond our delimited study area and so were excluded from the analyses. Species with a wide breeding distribution invariably had larger breeding than wintering ranges, but the opposite was true for species with limited breeding distributions. This outcome was at least partly due to underestimation of the breeding range of species with a small breeding area and an overestimation of the wintering range in species having a large wintering area. Our systematic survey of the Sahel revealed that bird species wintering in the northern and driest part of the Sahel actually wintered further north than indicated on published distribution maps, whereas species from the southern, humid zone wintered further south. The Sahel surveys indicate that the total population size of species breeding mainly in southern Europe, such as Masked Shrike Lanius nubicus, Western Bonelli's Warbler Phylloscopus bonelli, Subalpine Warbler Curruca iberiae + C. subalpina + C. cantillans and Rüppell's Warbler Curruca ruppeli, have so far been underestimated, but that population sizes of Common Redstart Phoenicurus phoenicurus and Common Whitethroat Curruca communis have probably been overestimated.

Effects on Birds of the Conversion of Savannah to Farmland in the Sahel: Habitats are Lost, But Not Everywhere and Not For All Species

Abstract: Many migratory bird species, several of which are in severe decline, and African residents spend the northern winter in the Sahel, by nature a huge savannah, half of which has been converted into farmland. We analyse the impact of such large-scale changes on birds. On average, woody cover is 38% lower on farmland than on savannah. More critically, farmers have drastically changed the vegetation communities of their farmland. In the arid and semi-arid zone, they partly removed bird-rich trees such as Umbrella Thorn Acacia tortilis and Desert Date Balanites aegyptiaca, yet further south they created a richer bird habitat by replacing the original woody species by Winter Thorn Faidherbia albida, a preferred tree species for Afro-Palearctic migrants (but less so for Afro-tropical residents). Still further south, two bird-poor trees, Shea Tree Vitellaria paradoxa and African Locust Bean Tree Parkia biglobosa, dominate farmland, causing birds, mainly Afro-tropical residents, to lose habitat. As a consequence of farming, arboreal migrants are confronted with habitat degradation in the northern arid zone and in the southern humid zone, but face more favourable wintering conditions in the sub-humid central zone. Ground-foraging birds are more abundant on savannah than on farmland; 24 bird species from this group, including three wheatear species and many residents, are more than twice as abundant on savannah. Conversion of savannah into farmland has mixed outcomes for ground-foraging birds, but were generally negative except for five species (including Western Yellow Wagtail Motacilla flava) which were more than twice as abundant on farmland than on savannah. Thus, the conversion of savannah into farmland represents a loss for many but not all bird species.

Birds and Bush Fires in African Savannahs

Abstract: Bush fires are widespread in African savannahs. Their impact on birds varies. Many insectivores temporarily profit from the insects escaping fire and smoke, whereas the burnt-through grass and herb layer facilitates feeding for some ground-foraging bird species. Nevertheless, bush fires have a direct, negative impact on many other ground-foraging birds. The average density of seed-eating birds in humid, African savannahs (annual rainfall >800 mm) was 15.9 birds/ha in unburned savannahs, compared to 3.3 birds/ ha (–72%) in recently burned areas. No such difference was found for insectivorous bird species. Eleven of the 13 common ground-foraging migratory bird species were not affected by bush fires in Africa because they spend the northern winter in the arid and semiarid zone, beyond the main bush-fire zone. In the long run, savannah-inhabitant birds profit from bush fires, simply because fires prevent open landscape from becoming overgrown with trees. However, the short-term implications of bush fires might be severe for seed-eating birds that rely on humid savannah, because of the more than 3 million km2 in Africa burned annually, most comprises humid savannah.

Living on the Forest Edge: Flexible Habitat Use in Sedentary Pied Flycatchers Ficedula Hypoleuca during the Non-Breeding Season

Abstract: Seasonality affects the availability of resources within the African non-breeding environment of migratory songbirds. We are generally unaware of how songbirds respond to such seasonal dynamics, especially at small spatial scales that are relevant for individual birds. In this study we focus on the question of how migratory songbirds use small scale variation in seasonality in their non-breeding environment. Therefore, we measured individual movements of European Pied Flycatchers Ficedula hypoleuca in relation to habitat differences in foliation in a non-breeding site in Comoé National Park, Ivory Coast. Through a combination of remote sensing and radio tracking we show that flycatchers change their habitat use during the second half of the non-breeding season, where at the start of this period flycatchers occupy both savannah and forest, whereas with progressing foliation, after savannah burning and with the onset of the first rainfall, they narrow their site use in favour of savannah. Further measurements of arthropod abundance show that this behaviour is related to increasing numbers of particular arthropod groups during foliation, which indicates that flycatchers might track seasonal changes in food availability by moving between habitats on a small spatial scale. We hypothesize that individuals reduce their susceptibility to seasonality by establishing territories on the forest edge, where they can access both savannah and forest habitat, and thereby explore a wider variety of resources under different circumstances. In conclusion, these findings indicate that small-scale heterogeneity likely plays a key role in the ability of flycatchers to cope with seasonal dynamics on a local scale.

Experimental Food Supplementation at African Wintering Sites Allows for Earlier and Faster Fuelling and Reveals Large Flexibility in Spring Migration Departure in Pied Flycatchers

Abstract: By travelling vast distances, migratory birds take advantage of earth's seasonality. Afro-Palearctic migrants can profit from lush spring conditions in temperate regions for chick rearing, but must also gain sufficient energy reserves to cross the Sahara. Rainfall during the dry season in Africa may influence the food available to birds to accumulate reserves. Conflicts of interests in resource exploitation at locations thousands of kilometres apart may occur if migrants encounter poor food conditions during these migratory preparations. Studying how wild birds adjust their fuelling and migration decisions to dynamic environments allows us to understand how flexible migrants can be, which is particularly important in an era of rapid change. We performed supplemental feeding prior to migration in individual Pied Flycatcher Ficedula hypoleuca wintering territories in Ivory Coast and remotely monitored their body mass change until they started their spring migration flight over the Sahara. We tested how access to extra food causally affects fuelling, departure mass and departure date. Seasonal fluctuations in natural arthropod availability prior to migration were monitored in two years, to explore how natural resource dynamics alters fuel accumulation. Birds that fully accessed extra food in March–April put on weight earlier and faster than birds without extra food supply, and departed 12 days earlier. Birds accumulated fuel loads that were higher than required for the Sahara-crossing, regardless of their access to extra food. Fuelling rates fluctuated in synchrony with natural conditions, as non-supplemented birds achieved the highest body mass gains at the time that natural arthropod availability peaked in the study area. Fuelling rates were lower in 2020, i.e. the year when the first rains after the dry season started late, than in 2019. Our study showed that Pied Flycatchers modulated fuelling rates – but not departure fuel loads – to food dynamics in West Africa, causing flexibility in the timing of departure. This strategy probably enhances a safe Sahara crossing, but may limit the possibilities of migrants to anticipate advancing spring conditions at breeding sites.

Savannah Trees Attract More Migratory Bird Species Than Residents, But Why?

Abstract: Arboreal bird species occurring in the wide transient zone between Sahara and tropical rain forest are unequally distributed across the rainfall zones. As this also holds for the woody plant species which they select for foraging, it is possible that birds are bound to specific rainfall zones because their preferred woody species are common there. But it may also be the other way around, i.e. that the distribution of birds is primarily determined by their selection of a specific rainfall zone, with the choice of particular woody plants being collateral. We made maps of the predicted distribution of birds based on their occurrence in different woody species (such as measured from field study sites) multiplied by the average density at which bird species forage in those woody plant species. We then compared these maps with the observed distribution of 13 bird species (7 Afro-Palearctic migrants and 6 Afro-tropical residents). This comparison shows that the distribution of birds is largely determined by the distribution of their preferred woody species rather than rainfall. However, there are small, but systematic differences between observed and predicted bird densities in the most arid and most humid parts of their distributions. Most migrants are commoner than predicted in the semi-arid and arid zone (100–600 mm rainfall/year) and most residents commoner in the humid zone. This was confirmed in a separate analysis of the densities at which these bird species forage in five common and bird-rich tree species occurring over a wide range of rainfall zones. There are no empirical data to support the idea that migrants and residents are spatially separated to avoid interspecific competition, so the question remains what migrants gain by their preference for trees from the (semi)arid zone. In the (semi)arid zones, preferred trees are as fully leafed in the dry season as the same trees farther south, but insectivorous birds in the arid zone had a higher capture rate in those trees, suggesting a larger supply of insect prey. In addition, the driest zones held far fewer avian predators than any other vegetation zone in the sub-Sahara, indicating a lower predation risk. We suggest that arboreal birds find better living conditions in the dry zones than in the more humid zones. But there is a trade-off: arid regions have a higher overall probability of very low rainfall years when trees lose their leaves or even die, than do the more humid regions. In those years, mortality among birds in the arid zones will be disproportionally high.

Sandwich Tern Feeds Juvenile on Wintering Grounds in Southern Namibia

Abstract: The duration of parental care varies widely among bird species. The crested terns Thalasseus spp. continue to feed their chicks at least sporadically for several months after fledgling. We recorded provisioning of a juvenile Sandwich Tern T. sandvicensis by its suspected parent at the wintering grounds in southern Namibia, more than 10,000 km away from the nearest breeding site and more than six months after fledging.

Foraging Ecology of Red-Billed Tropicbirds on Saba, Caribbean Netherlands, during Early Chick-Rearing

Abstract: Prey resources in oligotrophic tropical marine environments are often scattered and unpredictable. Tracking studies of tropical seabirds can provide critical information about ocean habitat affinities, prey choice and the utilisation of surrounding areas, which can be useful for conservation managers. Foraging studies of Red-billed Tropicbird populations in the Caribbean are scarce but increasing. We sought to expand on this by tracking chick-rearing adults using GPS devices and subsequently linking these tracking data to remotely-sensed environmental variables. We related our spatial data to opportunistic sampling of regurgitates in a globally significant nesting colony on Saba, Caribbean Netherlands. Diet samples were dominated by flying fish (Exocoetidae; numerical frequency: 70.73%), but prey items from the squid family (Loliginidae; 9.76%) and the families of flying gurnards (Dactylopteridae; 2.44%) and the ray-finned fish (Carangidae; 2.44%) were also identified, although we were unable to identify 14.63% of samples due to digestion. An additional goal of our study was to compare the foraging ecology of Tropicbirds on Saba with those nesting on St. Eustatius, located circa 25 km south-east. As expected, Tropicbirds nesting on Saba exhibited diurnal foraging patterns, travelling a maximum distance from the colony of 553.7 km, with an average trip length of 117.2 ± 144.6 km (±SD). Adults foraged in shallower, cooler waters with higher chlorophyll a concentrations and higher Exocoetidae species richness compared to travelling points. Despite the proximity of Saba and St. Eustatius, this is contrary to what was found for Tropicbirds nesting on St. Eustatius, where adults foraged in deeper waters with a low Exocoetidae species richness. However, Tropicbirds from Saba and St. Eustatius did exhibit some similarities in their foraging behaviour; specifically, foraging adults traversed multiple exclusive economic zones and marine protected areas, reinforcing our recommendation for nature managers in the Caribbean to create a transboundary network in order to effectively protect and conserve this species.

Second Breeding Attempts of Sandwich Terns in a Different Colony: Facilitated by Breeding Asynchrony between Colonies?

Abstract: Seabirds have long lives, reproduce slowly and usually do not attempt to relay within the same breeding season after nest failure. In Sandwich Terns Thalasseus sandvicensis, renesting has never been documented despite the large number of studies on their breeding ecology and recorded prospecting behaviour after nest failure. In 2020, we observed how three GPS-tagged Sandwich Terns moved 115 km after nest failure for a second breeding attempt. One of these birds hatched a chick from the second clutch successfully. We hypothesize that synchronous breeding usually prevents Sandwich Terns from trying again after nest failure and that the relays observed in 2020 were facilitated by the unusual situation of a successful breeding colony that started three weeks later than all other colonies in the Netherlands.

The Diet of the West African Crested Tern Unveiled by an Analysis of Otoliths Collected Over 20 Years along Its Main Breeding Sites

Abstract: This study reports on the diet of breeding West African Crested Terns Thalasseus albididorsalis on the basis of otoliths found in excrement (a mixture of pellets and faeces near nests). During the period 1998–2019, towards the end of the incubation period, excrement samples were collected from breeding colonies on islands, along the Atlantic coast from Mauritania to Guinea. In 31 samples with a total of 8956 otoliths we identified 53 fish families, 75 genera and 101 species. Twelve fish families occurred in more than 2% of the samples. The families of Haemulidae, Mugilidae, Pristigasteridae and Sparidae were particularly numerous (found in 10.5–15.7% of samples). The most numerous species were Bigeye Grunt Brachydeuterus auritus (9.8%) and West African Ilisha Ilisha africana (13.3%). There were marked differences in the diets of terns from the Banc d'Arguin (Mauritania), The Langue de Barbarie (northern Senegal) and all more southerly sites together (Senegal, The Gambia, Guinea Bissau and Guinea). On Ile aux Oiseaux, Delta du Saloum, Senegal, we found long-term changes in the diet of the terns with a marked decrease in Brachydeuterus auritus and the family Sparidae and a strong increase in Ilisha africana. Our analyses suggest that breeding West African Crested Terns are food generalists rather than specialists. This is surprising in view of the general trend in terns of the genus Thalasseus. There are indications that chick diets may differ from those of adults.

On the Wintering Ecology of Montagu's Harriers in West Africa: Itinerancy in Relation to Varying Annual Environmental Conditions

Abstract: Palearctic migrants wintering in Africa commonly use several sites throughout the winter, a strategy known as ‘itinerancy’. In this way, migrants track spatiotemporal variation in resources. Despite the importance of this strategy for migratory landbirds, we still lack detailed understanding of how variation in environmental conditions affects site use and the timing of movements between sites. We tracked 125 adult Montagu’s Harriers Circus pygargus from Western European breeding populations between 2005 and 2018 using satellite transmitters and GPS trackers. In total, data on 129 complete wintering seasons were obtained, including 33 individuals that were followed in two or more winters. Montagu’s Harriers were itinerant, using on average 3.3 wintering sites, to which they showed high site fidelity between years. The first sites harriers used after arriving in their wintering range were situated in the northern Sahel and were dominated by natural and sparse vegetation. Subsequent sites, situated further south in the Sahel, were mainly dominated by agricultural and natural habitats. Sites used by harriers had higher habitat diversity compared to random sites. Home range size and activity (time flying per day, daily distance) peaked at the last sites harriers used (i.e. the site from which they commenced spring migration). For individuals tracked in multiple seasons, we showed that home range size did not depend on vegetation greenness. However, the birds covered longer daily distances at the same site in drier years compared to greener (wetter) years. Importantly, the timing of the movements between wintering sites was affected by local environmental conditions, with individuals staying for shorter durations and departing earlier from first sites in drier years and arriving earlier at last sites in greener years. We conclude that within the context of a strategy of itinerancy, Montagu’s Harriers are faithful to the sites they use between years (spatial component), but flexible in the timing of use of these sites (temporal component), which they adjust to annual variation in environmental conditions.

Nestling Growth Pattern and Breeding Biology in the Eurasian Nuthatch Sitta europaea

Abstract: Avian growth has been traditionally examined using logistic, Gompertz or von Bertalanffy non-linear equations. Although many studies have analysed nestling mass increase and the factors affecting growth rates, there is little information on nestling growth curves for several avian groups, such as Nuthatches (Sittidae). Moreover, the breeding biology of Nuthatches is still poorly studied because of their reluctance to use nest boxes. Here we examined the growth pattern in Eurasian Nuthatch Sitta europaea nestlings, in addition to studying the breeding biology of this species, in a mixed deciduous forest from Central Portugal during one year. The nestling growth was well fit by the logistic and Gompertz models, whilst the von Bertalanffy equation was less accurate, especially when predicting the asymptotic mass and the mass in the first development stages. Nestlings attained their maximum mass around the age of 18 days, while the largest daily mass increase was achieved when nestlings were 8 days old. Weather conditions did not affect nestling growth. Regarding the breeding biology, the mean date of laying of the first egg was 3 April, the mean clutch size was 5.43 ± 0.78 (±SD) eggs and the mean number of fledglings per clutch was 4.00 ± 2.09. The proportion of eggs that produced fledglings in a nest averaged 72%. Clutch size and number of fledglings were negatively correlated with the laying date. These results contribute to our understanding of Eurasian Nuthatch breeding biology and nestling development, the latter providing useful data for the standardization of ornithological practices, such as determining the appropriate fledgling ringing period.

To See, Hear and Speak: How Counts of Birds in Individual Trees Help Address the Environmental Causes of the Sahel

Abstract: the three ‘wise monkeys’ originating in Japanese and Chinese philosophical traditions going back at least 1000 years. Mizaru was the monkey that covered its eyes, Kikazaru plugged both ears with fingers, and Iwazaru held its mouth shut with a clasping hand. Together they “saw no evil, heard no evil and spoke no evil”, a wonderful ambiguity, because is it really a virtue to withdraw? Or is it a virtue to see and hear attentively, and thus sense the state of the world around us; and then speak about it? This special issue of ARDEA is filled with papers painstakingly reporting the work of a small and dedicated team who set out to map the birds across the entire Sahel (an area the size of the USA!). Although mapping is inherently biased by the knowledge and cognitive facilities, as well as the interests, of the mappers (Malavasi 2020), within the limits of their sensibilities, and negotiating serious political and safety realities, the team tried to do this in temporally and spatially unbiased and methodologically robust and repeatable ways. The way that single trees and bushes disperse across the landscape of the Sahel, rather than connect-up into a dense forest, inspired a mapping approach that is both brilliant and unique. Rather than taking ‘an area’ as the spatial unit to measure bird abundance, the team began with ‘individual trees’ (Figure 1), with the plots in which these trees occurred being carefully pre-selected along trajectories that could be travelled “easily” (i.e. within the reach of a 4×4 vehicle). In their ensemble, the effort would give unbiased measurements of birds ánd trees across the entire Sahel, from Senegal in the west to Ethiopia in the east. This design also enabled assessments of the consequences of the steep latitudinal gradients in rainfall as one moves south from the Sahara sands towards the Sudan forests across 1000 km, and sometimes less. Theunis Piersma & El-Hacen M. El-Hacen

Distribution and Relative Density of Raptors in the Sub-Sahara during the Dry Season

Abstract: The sub-Sahara between the Atlantic Ocean and Red Sea and between 5°N (Guinean vegetation zone) and 20°N (southern Sahara) was visited on 466 days during 15 dry seasons (late September – early March) in 1996–2019. Using a combination of field methods, ranging from road counts to surveys of single sites (non-random and random-stratified), a total of 22,696 raptors of 62 species were identified. These were allocated to 1° latitude-longitude grid cells. Palearctic migrants accounted for 13% of the total. Two Afrotropical raptors were by far the most common, Yellow-billed Kite Milvus aegyptius (46%) and Hooded Vulture Necrosyrtes monachus (25%). Diversity and density were lowest in the arid and semi-arid zones but increased with increasing annual rainfall and vegetation cover. Palearctic migrants almost exclusively occupied the driest zones (100–500 mm rainfall per year), African raptors were commonest in the more humid zones. Migrants were concentrated in the western and eastern sections of the sub-Sahara, in longitudinal agreement with the main crossing points on either side of the Mediterranean for the large majority of Palearctic migrants. Comparatively few migrants were encountered in the central Sahel (Mali-Niger-Chad), suggesting that most Palearctic raptors remained either in West or in East Africa upon entering the continent. Even harriers Circus spp., known to cross the full width of the Mediterranean Sea, showed a distinct East Africa bias in their distribution. Afrotropical raptors were more evenly distributed across the width of the sub-Sahara within the 100–1000-mm rainfall zone.

Shrub-Dwelling Birds in the Sahel Forage Less Often on the Ground in Grazed Areas

Abstract: Shrub-dwelling birds may resort to ground-foraging in the Sahel when opportunities are favourable. Several arboreal and semi-arboreal passerines, both African and European, were frequently recorded foraging on the ground, but not in heavily grazed areas. Grazed, dry savannah probably has fewer insects on the ground, which is often devoid of vegetation in the dry season. Shrub-dwelling birds foraged more frequently on the ground in the eastern Sahel, where grazing pressure is lower. In the Sahel grazing pressure increased fourfold since the 1960s, presumably reducing opportunities for arboreal bird species to facultatively forage on the ground. Due to increased grazing pressure, Common Whitethroats Curruca communis and other shrub-dwelling passerines may have lost a specific niche within their foraging habitat. This has compounded the greater losses associated with declines of woody vegetation during the drought years since the late 1960s.

First Report of Polymelia in a Passerine Bird, the Pied Flycatcher

Abstract: Polymelia is a congenital defect characterized by an excessive number of limbs. Leg malformations are unusual and have rarely been reported in birds. The ultimate mechanism that regulates this type of abnormal development is not yet well understood. Here we report polymelia in a nestling of the European Pied Flycatcher Ficedula hypoleuca. We have ringed more than 12,100 Pied Flycatcher nestlings since 1991 and have never observed this malformation before. To our knowledge, there is no study indicating polymelia in any bird of the order Passeriformes. We observed and studied a Pied Flycatcher nestling that had two extra limbs fused at the pelvis. We used X-rays, Computerized Tomography (CT) scan and Magnetic Resonance Imaging (MRI). We describe the observed malformations and discuss potential causes. The malformation could be due to an embryonic cause, such as a parasitic twin (pygopagus parasitic) or a genetic mutation.

The Effect of Environmental Conditions on Captures, Survival and Breeding Success of a Winter-Breeding Seabird.

Abstract: Few studies have explored local and global environmental effects on the demographic rates of small seabird species such as storm-petrels. We analysed ringing data (from 2011–2022) to investigate the demographic parameters of a breeding population of Band-rumped Storm-petrels Hydrobates castro, which nest on Farilhão Grande Islet, Berlengas archipelago, Portugal. We used capture-mark-recapture analyses to estimate annual capture probabilities, apparent survival and abundance. The effects of environmental and capture effort-related variables on demographic parameters were then evaluated. The mean annual survival estimate was low (0.68 ± 0.02 SE) in comparison to other storm-petrel species, but this estimate substantially increased to 0.79 ± 0.02 after removing transient individuals. During the study period we determined breeding success in 10 years. In four of these years we also monitored breeding attempts with automatic cameras (2014–2017). Breeding success was low (0.56 ± 0.12 fledglings per active nest) with some observed cases of predation by Yellow-legged Gull Larus michahellis of both adults and chicks. The size of the Band-rumped Storm-petrel population was estimated at 1511.0 ± 266.3 SE individuals in 2017 when the mist netting effort was highest. There is no evidence that this population experienced a large decrease over the last 27 years and the trend from the previous 11 years seems to indicate a fluctuation rather than a clear decrease. The North Atlantic Oscillation Index showed a positive effect on the number of captured birds, while there was a negative effect from moonlight and local winds. We conclude that mist netting is a valuable method for long-term demographic studies on ground-nesting seabirds in which nests are difficult to access, but estimates of demographic parameters are influenced by environmental and capture effort-related variables.

Food Provisioning and Body Mass of Nestling Meadow Pipits and Cuckoos

Abstract: Common Cuckoos Cuculus canorus are obligate brood parasites, laying eggs in nests of other species. Cuckoo nestlings are often thought to be insatiable, compared to host broods. However, in Reed Warblers Acrocephalus scirpaceus and a few other species, cuckoo nestlings are fed at most only as frequently as a host brood. To add to the small body of knowledge on feeding frequencies and body mass development of Cuckoo nestlings, I studied Cuckoos using another host, the Meadow Pipit Anthus pratensis, in the Dutch dunes (2019–2021). By filming feeding parents (hosts) I determined the provisioning frequency for broods of Pipits and for nestling Cuckoos. I found that nestling Cuckoos receive on average fewer feeds per hour than broods of Meadow Pipits. Furthermore, overall Cuckoos received as many feeds during their 22-day nestling period as a brood of Pipits during their 13 days in the nest. At 13 days of age (day 1 is the day of hatching), the single Cuckoo was as heavy as a brood of four Meadow Pipits. At 22 days, the maximum weights of the nestling Cuckoos were greater than the Meadow Pipit broods and varied between 91 and 105 g. Thus, although Cuckoos are being fed less frequently, young Cuckoos are heavier at fledging than a whole brood of Pipits. This could be because Cuckoos are fed larger prey. Alternatively, Cuckoos may require less food because their thermoregulatory costs could be smaller: they have almost black skin which absorbs solar radiation efficiently, do not have to compete with siblings in the nest and, once older and feathered, have a smaller surface-to-volume ratio than a Pipit brood of four nestlings.

Dynamics of Naïve White Storks Ciconia Ciconia Migrating at the Central European Migratory Divide during 1947–2018

Abstract: Flexibility in migratory behaviour may have significant consequences for the adaptability of birds to ongoing environmental changes. Shifts in arrival and departure dates, migration distance and even direction of migration have all been demonstrated over the last few decades in a variety of species, mostly passerines. Less well understood is the spatio-temporal dynamics of migration at so-called migratory divides, i.e. contact zones of populations with different migratory strategies. We used data on 58 White Storks Ciconia ciconia ringed as nestlings in a part of the Central European migratory divide in the Czech Republic during the period 1947–2018 and recovered during their first winter season in October to February, to test the hypotheses of (1) shortening migratory distance over the years and (2) a shift in the proportion of adopted migratory strategies being a proximate factor of changes in migration distance. Using regression analysis, we provided evidence of gradual linear decrease over the years in mean recovery distance of ringed nestlings during their first winter. Results of multinomial regression analysis showed that this decrease has been proximately driven by an increase in the proportion of storks recovered within 500 km from their natal site, a decreasing proportion of storks recovered on the eastern migratory flyway and an increasing proportion of storks recovered on the western migratory flyway. Evidence for Czech Storks migrating via the western migratory flyway first appeared in 2003. The apparent shift of young Storks to migrate via the western flyway from the Czech Republic is most likely a result of immigration by individuals from growing western populations. This leads to diluting of populations migrating via the eastern flyway with migrants using the western flyway. Our study provides evidence that behavioural flexibility may drive dynamics of migration at the migratory divide, with changes of similar magnitudes as those driven by genetic change, as reported in passerines.

Responses of Yellowhammer Emberiza citrinella, Pine Bunting E. leucocephalos and Their Hybrids to Playbacks of Con- and Heterospecific Songs and Calls in a Hybrid Zone

Abstract: Yellowhammers Emberiza citrinella and Pine Buntings E. leucocephalos hybridize in a c. 2500 km wide sympatric zone in northwest Asia. In the centre of this zone, over the last 50 years males with Yellowhammer and intermediate (i.e. hybrid) phenotypes have been replacing the Pine Bunting phenotype. In the southern part of the zone by contrast, the Pine Bunting phenotype is still common. In this study, we asked if there is a behavioural asymmetry between males depending on their phenotypes and whether this asymmetry can explain phenotypic compositions observed in different parts of the hybrid zone. This study was performed in 2017–2019 in Altai Republic, Russia, in the southern part of the hybrid zone. Songs of Yellowhammer and Pine Bunting are generally similar, although they differ in details. The species also differ in usage of call repertoires: the ‘see’ call is more characteristic for Yellowhammer, while the ‘zieh’ call is used more frequently by Pine Bunting. We performed playback experiments using contrasting stimuli to evaluate responses towards (1) Pine Bunting song vs. Yellowhammer song and (2) songs coupled with ‘zieh’ calls vs. songs coupled with ‘see’ calls. Experiments showed that Yellowhammer and hybrid males were more aggressive towards other males than were birds with the Pine Bunting phenotype. Therefore, the latter might be at a disadvantage in territory acquisition and maintenance. At the same time, males of any phenotype responded more strongly to Yellowhammer playback than to Pine Bunting playback, and to the ‘see’ call than to the ‘zieh’ call. This might counterbalance the competitive ability of different phenotypes in territorial competition, thus allowing Pine Bunting males to avoid being displaced by more aggressive Yellowhammer and hybrid males in the southern part of the hybrid zone.

Contrasting Trends and Dynamics in the Autumn Aggregation of White-Tailed Eagles at Two Fishponds in Lithuania

Abstract: The White-tailed Eagle Haliaeetus albicilla may aggregate in areas with high fish supply during their autumn movements. Here, we report on data collected between 2008 and 2019 concerning numbers of White-tailed Eagles visiting fishponds located in NE Lithuania, to the east of the Baltic Sea during the autumn months. For comparison, between 2016 and 2018 we also surveyed White-tailed Eagles at another, similarly managed fishpond, located 110 km away in SE Lithuania. The number of White-tailed Eagles at the fishponds in NE Lithuania in October through December increased nearly three-fold between 2008 and 2019. The long-term increase occurred especially in November–December and much less in October, and this, together with sudden increases in numbers in certain years, suggests possible changes in their movement patterns. We found similar numbers of assembling eagles in the two fishponds in October, but in November there were significantly more eagles at the fishponds in NE Lithuania. The size of groups in autumn and the seasonal dynamics therein may differ at different stopover sites that are similar in size, harvesting practice and located in the same region. We suggest this could be due to differences in the surrounding environment.