Showing papers in "Ibis in 2008"

The timing of birds‘ breeding seasons

Abstract: Summary Examination of survival rdtes of nestlings and fledglings of some species show that there is a strong tendency for those young which are hatched earliest in the season to have the greatest chance of surviving to breed. Since natural selection so strongly favours parents who leave many surviving young, the question arises as to why other birds breed later than the date at which they could most successfully raise their young. It is suggested that the food supply for the breeding females immediately prior to the breeding season may limit their ability to form eggs and the females may thus not be able to lay at the time which would result in young being in the nest at the best time for raising them, but as soon after this time as the female is able to produce her eggs. Not all species are likely to be prevented, by food shortage, from breeding at the best time for raising young and the groups of birds most likely to be affected are discussed.

The importance of certain assemblages of birds as “information‐centres” for food‐finding

Abstract: Summary Evidence is presented to support the hypothesis that communal roosts, breeding colonies and certain other bird assemblages have been evolved primarily for the efficient exploitation of unevenly-distributed food sources by serving as “information-centres”. Predation-pressure is regarded as being the most important factor “shaping” the assemblages. The shaping involves the choice of inaccessible or otherwise safe sites, optimum dispersal, mutual awareness of attack and joint defensive tactics, and serves to minimise the vulnerability to predation which would otherwise result when birds mass together in conspicuous, and often predictable centres.

Patterns of growth in birds

Abstract: Summary Parameters used to characterize the course of growth are described, and calculated growth parameters are presented for 105 species of birds of many taxonomic groups from a wide range of geographical localities. Growth parameters are found to exhibit as much as 20% variation within a species with respect to geographic locality and time of the nesting season. There is also considerable local variation, irrespective of season and locality, which is related to nutrition and perhaps to an inherited variability. The application of curve-fitting as a method of analysing intraspecific variation is discussed briefly, and the importance of comparative growth studies is emphasized. Growth patterns are correlated with other parameters of the life-history to evaluate the extent of diversity in the course of growth. Low rates of growth and prolonged growth periods occur primarily in species large for their families and in oceanic species. In most others, high rates of growth are maintained for longer periods of time. The shape of the growth curve is not related to the mode of development (i.e. whether precocial or altricial). Overall relative, or weight-specific growth rates, as measured by the constants of fitted growth equations, are most highly correlated with the adult body size of the species, changing as the -0–278 power of adult body weight. Smaller variations in the rate of growth appear to be correlated with differences in nesting success; open-nesting passerines grow faster than hole-nesting species of a similar size. Growth rate is further correlated with brood size. Oceanic species with single egg clutches and tropical land-birds with small clutches have low growth rates. The asymptote of the growth curve of the young (in relation to the adult weight) is related to the foraging behaviour of the adults. Aerial feeders generally have high asymptotes while those of ground feeding species are usually below adult weight. These differences are related to the need in the former for well-developed flight at the time of fledging. The diversity of growth patterns is related to evolutionary trends which are the result of (1) selective forces acting at stages of the life-history cycle other than development, (2) factors which affect the survival of offspring during the growth period, and (3) adjustments made to balance the energy budget of the family group. The last trend is discussed in detail in relation to the correlations found in the analysis. Two hypotheses are presented. Firstly, in species which cannot gather enough food to support even one young at a normal growth rate, the pace of development is reduced to decrease the energetic requirements of the young. Secondly, in species with small clutches, where adjustments to feeding capacities are not readily made by changing brood size, growth rate may be adjusted to accomplish this. The lack of critical energetic data to test these hypotheses is emphasized as a major deficiency in our understanding of the breeding biology of birds.

The regulation of numbers of tropical oceanic birds

Abstract: SUMMARY The ways in which the numbers of tropical sea-birds might be limited are considered; it is argued that food is the only factor likely to be generally effective in limiting numbers, but it seems improbable that food shortage could exert a density-dependent control of the mortality of the birds outside the breeding season. Wynne-Edwards' hypothesis that colonies of sea-birds are able to keep their own numbers below the level at which food shortage would become acute, primarily by exerting control on the output of young, is rejected as unproven and improbable. It is suggested that colonies of tropical oceanic birds deplete the food in the waters round them, and that as the populations increase competition for food becomes more intense, and relatively fewer adults succeed in raising chicks. This would provide a density-dependent control of the output of young and could regulate the numbers of the birds. The peculiarities of long-lived sea-birds (e.g. clutches of one, long fledging periods, deferred maturity) which Wynne-Edwards suggests are adaptations evolved in order to lower the reproductive rate until it balances the mortality, apparently could not be evolved as such; they are more probably adaptations enabling the birds sometimes to raise single chicks in spite of competition for food that makes it impossible to raise more than one. It is considered that variation in the age of first breeding provides an important supplement to variation in reproductive success in regulating the numbers of long-lived tropical sea-birds. The possible applications of this hypothesis to sea-birds breeding in higher latitudes are briefly considered, as are its implications in relation to conservation and exploitation of populations of sea-birds.

Do tropical birds rear as many young as they can nourish

Abstract: Summary. 1 The thesis that birds lay clutches of eggs which will produce as many nestlings as they can, on the average, adequately nourish, is considered in relation to the Central American avifauna. 2 Birds whose usual rate of bringing food to the nest is slow can greatly augment this rate if, after an exceptionally long period of neglect, they find their nestlings unusually hungry (examples are given for two species of antbirds), or if an additional nestling is placed in the nest (an experiment with a tanager is described). 3 In four out of five cases, birds of four kinds in which the clutch consists almost invariably of two eggs succeeded in rearing three nestlings. 4 In numerous species of hummingbirds, manakins, cotingas, American flycatchers, Icteridae, etc., the male does not help to feed the nestlings; yet these birds nearly all rear broods of two, which in the same habitats is the prevalent size among species in which both parents attend the nest. If the latter were rearing as many young as they could properly provide for, we should expect the unmated females to lay clutches only half as big. Among flycatchers, when the female alone feeds the nestlings, their period in the nest is not significantly longer than at comparable nests of species in which both parents feed. 5 Also in nidifugous species, of which the chicks pick up their own food under parental guidance, clutches are far smaller in the Tropics than in the North. 6 On the basis of the time spent away from the nest by 18 incubating finches of 9 species, it is estimated that on the average 4 hours per day suffice each parent to find all the food it needs, preen, bathe, and perform other necessary activities. This would leave each parent, of the tropical species, about 8-5 hours to devote to the feeding of the nestlings; both together would have 17 hours per day. Even allowing 5 bird-hours for each nestling, the two parents together could attend 3; yet 7 of the 8 tropical species considered regularly lay sets of only 2 eggs. 7 Even when the climate appears to be favourable through much or all of the year, the breeding-season of many tropical birds is short and the number of broods small. The fact that some individuals breed successfully at seasons when most members of the same species are resting from reproduction, points strongly to the conclusion that these birds do not attempt as many broods as they might successfully rear. Neither in the size of their broods, nor in the number per year, do tropical birds in general appear to rear as many offspring as their own powers and the environment would permit. Their rate of reproduction seems to be adjusted to their average annual mortality rather than pushed to the limits of their strength. 8 Two species of Tyrannidse, Myiozetetes similis and M. granadensis, are similar in appearance and habits and often build their nests in the same trees, but differ in their rate of reproduction. In the midst of their ranges, it does not appear that the more prolific M. similis has any advantage over its congener. 9 In a hypothetical species which with a clutch-size of two keeps its range occupied at a high or optimum density, the fate of a three-egg mutant is considered. It would be difficult for the more fertile genotype to displace the well-established two-egg strain, unless some catastrophe caused a severe reduction in the density of the population. Such catastrophes are rare in the humid Tropics. 10 The situation is radically different in a species expanding into a new area, or in one increasing rapidly in numbers after a great reduction in density of population. In these cases intraspecific competition is at a minimum, and the more prolific strains will tend to predominate. At high latitudes, recurrent drastic reductions in density as a result of great cold, famine, or disasters during migration, are followed by periods of free expansion; and under these circumstances it is likely that the rate of reproduction will be held to near the maximum which combined internal and external conditions allow. Lack's views appear to fit the facts in relation to the northern birds which he investigated, but seem to be in accord neither with observations nor theory when applied to the birds of humid tropical areas, where ecological catastrophes are at most rare and local.

The learning of song patterns by birds, with especial reference to the song of the chaffinch fringilla coelebs

Abstract: SUMMARY 1. The songs and other vocalisations of birds are of theoretical interest to zoologists and psychologists (1) as social communication systems and as a “language”; (2) as specific and inter-specific recognition marks; (3) as a problem in the inheritance and genetical control of elaborate behaviour patterns; and (4) as a problem in the acquistion of complex behaviour patterns by individual learning. The last two constitute the main objects of this work. 2. The Chaffinch Fringilla coelebs was chosen as the main subject of investigation because its song displays both inherited and individually learned components, the relation between which is of particular interest. It is considered that neither a chain reflex type of theory nor one of reflex conditioning in the ordinary sense will provide satisfactory explanation of song production. 3. The characteristics and normal limits of variation of song of wild F. c. coelebs and F. c. gengleri are described and illustrated. There is no certain means of distinguishing the songs of these two subspecies but fairly consistent local differences occur. A wild male Chaffinch usually has more than one song type and may have as many as six. Full song is practically unknown in the female. 4. The song of insular species and sub-species and of artificially isolated populations tend to be simpler and less variable than examples from the Continent, possibly because, in a less dense population in an ecologically simpler environment, an individually distinctive territorial proclamation will not be so important for breeding success. 5. Experiments with F. c. gengleri were carried out in aviaries, bird rooms and cages. None of these techniques had any seriously adverse effect on the drive to sing. The onset of song could be controlled experimentally by crowding, by artificial control of daily illumination in a light-proof room and by injection of testosterone propionate. 6. Birds caught as juveniles in their first autumn and isolated until the summer following produced nearly normal songs that are almost as elaborate as are those of wild Chaffinches. If such birds, instead of being individually isolated, are kept together in groups in such a manner that they can hear only the songs of the members of their own group, these birds (as a result of counter-singing) copy one another and so come to produce a fairly uniform community pattern. Close matching of the final phrase of the song is particularly evident. 7. Birds which have been hand-reared in auditory isolation from the fifth day of life produce extremely simple songs which represent the inborn component of the specific song. If such Kaspar Hauser birds are themselves grouped together in isolated communities from the third to the thirteenth month of life, each group will-during the period February to May—build up, by mutual stimulation and imitation, complex but highly abnormal songs quite dissimilar from those of normal wild Chaffinches. From this it is clear that in the wild young Chaffinches learn some features of the song from their male parents or from other adults during the first few weeks of life. But most of the finer detail of the song is learnt by the young bird when, in its first breeding season, it first comes to sing in competition with neighbouring territory holders. There is little doubt that this is the way in which local song-dialects are built up and perpetuated. Full Chaffinch song is thus an integration of inborn and learned song patterns, the former constituting the basis for the latter. 8. The subsong of the Chaffinch is of low intensity, entirely different in structure from the full song, and contains a much bigger range of frequencies. It seems to have no communicatory function. It is characteristic of the early spring of first-year birds, and also of low but increasing production of sex hormones in birds of all ages. It provides in some degree the raw material out of which, by practice and by the elimination of unwanted extremes of frequency, the full song is crystallised. 9. The use of call notes, which are innate, as components of subsong and full song is described. Call notes are more evident in the songs of Kaspar Hauser birds than they are in the songs of normal birds. This is presumably an expression of the reduced auditory experience of the hand-reared isolated birds. 10. Chaffinches are not “imitative” birds in that they do not normally copy anything but sounds of Chaffinch origin. Once a Chaffinch has heard Chaffinch song as a young bird in the wild, it appears to have learnt enough about it to refuse to copy any sound pattern which departs far from the normal, i.e. it will learn only the fine individual variations of the songs of other Chaffinches. So, in the wild, Chaffinches practically never, in their full songs, imitate anything but other Chaffinches. Kaspar Hauser birds will learn songs of far greater abnormality provided the tonal quality is not too different from that of Chaffinch song. Voices as “abnormal” as that of a Canary may be learned by hand-reared birds and very occasionally by wild birds but when this happens the alien notes are kept as components of the non-communicative subsong only; the full song is not contaminated with them. If the tonal quality is sufficiently close to that of the Chaffinch, as with the song of the Tree Pipit and as with artificial songs constructed from genuine Chaffinch notes and played to the hand-reared birds by means of the “Song Tutor”, they will be learned at least to some extent and the learned pattern will become part of the bird's full song. 11. Counter-singing that occurs between birds in adjacent territories is an important factor in stimulating and restricting the imitative abilities of Chaffinches. When a Chaffinch has acquired more than one song type, each song outburst consists of a sequence of one song type followed by a sequence of another. This can be largely understood on the basis of the inter-action of the self-stimulating and self-inhibitory effects which are a concomitant of each song type. When songs are played back to a Chaffinch we find that those songs which it uses most frequently itself are the most effective in evoking song. A Chaffinch in the wild will thus tend to reply to a neighbour with that song of its own repertoire which most nearly resembles the song of its rival. 12. It is suggested that with the more “imitative” finches such as Bullfinch, Hawfinch and to some extent Greenfinch, the song, while functional in co-ordinating the breeding cycle and behaviour of the mated pair, is of less importance as a territorial proclamation. Thus in some respects it resembles the subsong rather than the full song of the Chaffinch and similarly contamination with alien notes can be tolerated to an extent which might be highly disadvantageous in a highly territorial song. A preliminary study of a number of species of “Buntings” shows that many of the species, e.g. Reed and Corn Buntings (Emberiza schoeniclus and E. calandra) have songs which are highly stereotyped and completely innate. The song of the Yellow Bunting E. citrinella appears to consist of an integration of innate and learned components much as in the Chaffinch. The Buntings as a whole have highly territorial songs. 13. Apart from a very few and partial exceptions, Chaffinches can only learn song patterns during the first 13 months of life and towards the end of this time there is a peak period of learning activity of a few weeks during which a young Chaffinch may learn, as a result of singing in a territory, the fine details of as many as six different songs. This special period of high learning ability is brought to an abrupt close by internal factors. This restriction of learning ability to a particular type of object and to a sharply defined sensitive period recalls the phenomenon of imprinting. 14. The inborn recognition and performance of the specific song pattern by the Chaffinch involves (a) duration of approximately 2 1/2 secs., (b) interval between songs of approximately 10–20 secs., (c) tonal quality (though this last may have been learned by the bird's experience of the qualities of its own voice). The readiness with which the bird learns to divide its song into three sections and learns to attach a simple flourish to the end as an appropriate termination suggests that there may be a very imperfectly inherited tendency to respond to and perform these features of the normal song which is revealed as soon as the singer is stimulated by hearing another bird.

Evolutionary aspects of fruit-eating by birds

Abstract: SUMMARY In spite of a considerable literature on fruit-eating, the general evolutionary implications of fruit as a source of food for birds have been neglected. A preliminary attempt is made to explore the evolutionary and ecological consequences of fruit-eating, considered as a mutual interaction between parent plant and dispersal agent. The relationship considered is that obtaining between fleshy fruits and the “legitimate” fruit-eating birds which digest the fleshy part of the fruit and void the seed intact. Evolutionary aspects of seed-eating are also briefly discussed. The “strategies” adopted by fruits for dispersal by birds result in the production of abundant food supplies which are easy of access and exploitable by many species of birds. By contrast, the predation of birds on insects leads to a heterogeneous, sparse and cryptic food supply, to exploit which many different hunting techniques are necessary. Two important evolutionary developments in birds are attributed to these differences in food supply: there tend to be more species in families of insectivorous than of frugivorous birds, and lek behaviour in tropical forest has evolved in predominantly frugivorous birds. The seasonal succession of fruits in temperate latitudes is discussed, and contrasted with the situation in the tropics, using examples from Europe and Trinidad. In general, the succession of ripe fruits in Europe seems to be adapted to the seasonal shifts of the bird populations, and the more nutritious fruits tend to have a more southerly distribution and to ripen later than the more succulent fruits. In the tropics the distinction between nutritious and succulent fruits seems to be largely one of habitat. The constant succession of ripe fruits throughout the year in the tropics probably depends on competition for dispersal by frugivorous birds, which thus ensure the maintenance of their own food supply. This may be regarded as a symbiosis at the level of the ecosystem.

Habitat fragmentation: island v landscape perspectives on bird conservation

Abstract: Fragments of habitat are often viewed as islands and are managed as such; however, habitat fragmentation includes a wide range of spatial patterns of environments that may occur on many spatial scales. Fragments exist in a complex landscape mosaic, and dynamics within a fragment are affected by external factors that vary as the mosaic structure changes. The simple analogy of fragments to islands, therefore, is unsatisfactory. Understanding how birds respond to these complexities of fragmentation requires mechanistic studies focused on habitat selection and movement behaviour. Conservation efforts must be based on viewing fragmentation as a range of conditions that occurs in a landscape mosaic, and management should be directed toward the mosaics rather than focusing solely on reserves.

Applications of stable isotope analyses to avian ecology

Abstract: In the past 20 years the use of stable isotope analysis has become increasingly common in ecological studies. In fact, in some instances these techniques have yielded remarkable insights into the foraging preferences and migrations of birds. Despite these advances and the potential of the approach, it is possibly still not as widely used as might be expected. In this paper we aim to illustrate the potential of the approach in the hope of encouraging more avian ecologists to think again about how these techniques might provide insights in the systems on which they work. We discuss some of the principles behind the approach, and review some of the more recent ornithological studies that have used stable isotope techniques to trace trophic pathways or infer migratory origins. We follow this by discussing some of the latest ideas on how stable isotopes may be used to generate community metrics and close by detailing the important assumptions and caveats that should be considered before undertaking any studies using this technique.

The contribution of artificial nest experiments to understanding avian reproductive success: a review of methods and conclusions

Abstract: Artificial nest experiments have been used in approximately 80 recent studies to identify a wide range of factors influencing reproductive success of birds. Despite the same factors being examined in different studies, few consistent patterns have emerged from these experiments. Analysis of reproductive success at artificial and natural nests reveals that reproductive success measured with use of artificial nests is frequently underestimated. In addition, several studies suggest that different species of predators may be differentially attracted to the two types of nest. These results call the utility of artificial nests into question as a tool for elucidating factors influencing the success of natural nests. Analysis of the methodology used in artificial nest experiments reveals that in general there is poor correspondence between the appearance of experimental setups and the natural systems they attempt to model. If future experiments with artificial nests are to contribute to theory, the quality of experiments must improve and the assumptions underlying the artificial approach must be tested. Collecting this information will be relatively simple compared with the time-consuming task of experimenting with natural nests. Until these data are collected, conclusions about nesting ecology derived from the use of artificial nest experiments should be treated as preliminary.

The biological significance of the territories of birds.

Abstract: Summary. 1 This paper forms an introduction to a symposium on the territorial behaviour of birds. 2 Following Noble (1939), territory is defined as “any defended area”. A simple classification of breeding territories is given, and the diversity of territorial behaviour emphasized. 3 Territorial behaviour can be analysed into at least two components—restriction of some or all types of behaviour to a particular area, and defence of that area. These are often accompanied by self-advertisement. 4 The aggressive behaviour which arises in different situations depends primarily on the same mechanisms, though the fighting may be specialized to serve different functions (e.g. to defend different objects). This justifies the broad definition of territory given above. 5 The fighting involved in territorial defence is associated with tendencies both to attack and to flee from the rival. 6 The nature of the evidence concerning the biological functions of territorial behaviour is considered. In previous discussions of this subject the term “function” has sometimes been used to refer to any advantageous consequence of the behaviour, and sometimes restricted to consequences through which selection in favour of the behaviour can act. 7 Various possible functions of territory are considered. (a) Circumstantial evidence suggests that the familiarity with the area which results from site attachment may assist feeding, escape from predators, etc., and may also increase fighting potentiality. (b) There is strong evidence that territorial behaviour, in addition to producing over-dispersion, can regulate density in favoured habitats. There is no direct evidence that territory limits the total breeding population in all habitats. (c) In many species, it facilitates the formation and maintenance of the pair-bond. (d) It may also reduce interference in various reproductive activities by other members of the species. (e) Defence of the nest-site is an important consequence of territorial behaviour in many species, and the aggressive behaviour is often clearly specialized to this end. (f) In a few species territory is primarily concerned with food, but in most the food value of the territory is not significant. Even in the species which feed on their territories, the territorial behaviour is not specialized for the defence of food objects:in these cases it is possible, but unproven, that the territorial behaviour does help to ensure an adequate supply of food for the young. (g) In some species the over-dispersion produced by territorial behaviour may reduce predation, though direct evidence that this is the case is not available. (h) Maintenance of a territory may reduce the despotism of other males, but this does not explain the function of territorial aggressiveness. (i) Territorial behaviour may reduce disease, but this is unlikely to be a significant consequence except in some colonial species. (j) It is unlikely that the prevention of inbreeding and the promotion of range extension are significant consequences of territorial behaviour. 8 The functions of territorial behaviour are extremely diverse, and the quality of the evidence available for assessing them is little different from that available to Howard. Since territorial behaviour has consequences both harmful and advantageous to the individual's chances of ultimate reproductive success, and since the inter relations between the selective forces governing behaviour, structure and physiology are extremely complex, simple answers about the function of territory cannot be expected.

The level of reserve protein as the proximate factor controlling the timing of breeding and clutch-size in the red-billed quelea quelea quelea

Abstract: SUMMARY Changes in the fat and flight muscle protein reserves of adult Red-billed Queleas Quelea quelea were followed in two colonies in Tanzania and Botswana. At the start of a breeding attempt the protein reserves were higher that at any other time of the year, particularly in females which had heavier flight muscles (non-fat dry weight) than did the (larger) males at that time. The pre-breeding increase in the labile component of the muscle protein (the actual protein reserve) is calculated at 80% for females, but only 14% for males. The fat reserve was only increased slightly at the beginning of the breeding attempt. In both sexes, though for different reasons, the protein and fat reserves fell rapidly during the first few days of the attempt, in some individuals to dangerously low levels. During the incubation period there was rapid recovery while the situation during the rearing period appeared to vary between colonies. It is proposed that the proximate control of breeding is provided by the individual's own body condition, and particularly the state of its protein reserves. No environmental releasers are required for the birds to breed at the appropriate time of the year. Individual females producing two, three and four egg clutches differed in the rate at which their reserves fell during egg formation. The results are used to support the view that in this quelea the actual clutch-size produced on any occasion is the largest the female can produce before becoming too enfeebled. Thus, clutch-size appears to be determined phenotypically downward from a maximum which is indirectly under genetic control.

Flocking and feeding in the great tit parus major–an experimental study

Abstract: Summary Some aviary experiments designed to investigate the survival value of flocking in the Great Tit Parus major are described. An individual searching for a source of food is less likely to find it if he is on his own or in a pair than when in a flock of four. This is a result of local enhancement: with more pairs of eyes searching, some member of the flock is likely to come across the food sooner than would a lone bird, and once one finds the food, others in the flock more or less immediately assemble at the food source. Once the birds have found a source of food, dominance interactions play a role in determining which individuals obtain the most food. If one bird finds a single item of food, both it and the other members of the flock rapidly alter their searching behaviour, concentrating their attention in the general area and /or type of place where the food item was found. Even if a bird searches in a place and does not find food, the others in the group show some increase in searching effort in the same area or type of place. The extent to which they do this depends on whether they are used to finding food in clumps or dispersed. The relevance of these findings to birds in the wild is discussed. It seems likely that local enhancement does occur in natural flocks, and the flocking in Great Tits is of benefit to the participants because they increase their effectiveness in finding food. However, the question of the survival value of mixed-species flocks of titmice remains unresolved.

Patterns of growth in birds. ii. growth rate and mode of development

Abstract: Summary This analysis was initiated to examine the relationship between the rate of growth in birds and their development of mature function The literature was surveyed for data on growth and development, and the growth curves of 81 species were chosen for the analysis Growth curves of most species were fitted with the Gompertz equation, and the rate constants of the equation were used as an index of the growth rate For those species whose curves were fitted better by other equations, with a slightly different form, appropriate conversion factors, derived in this paper, were employed Among species with similar modes of development, growth rate decreases with increasing body weight in an allometric manner, with slopes of –026 to –042, depending on the group Between groups, the rate of growth in body weight was found to be closely associated with the rate of development of function, in particular, the acquisition of flight Among those species that can walk at an early age, but acquire flight relatively late, the rate of growth depends primarily on the relative size of the musculature of the lower extremities Data are presented to refute the hypotheses that growth rate is adjusted to nestling mortality, or that the energy requirements of the young (and hence their growth rates) are balanced against brood size It is concluded that most species grow at some physiologically maximum rate, but as yet it is not possible to distinguish between limitation of growth rate at the level of the organism or at the level of the tissue

Tits and their caterpillar food supply

Abstract: In this paper I review what is known about the relationship between the tits' clutch-size and the timing of their breeding season and the numbers of their caterpillar prey. The results come from the Edward Grey Institute's long-term study of Great and Blue Tits in Wytham Wood, near Oxford. This study was started in 1947 by Drs D. Lack and J. A. Gibb (Lack 1966, Perrins 1979)- These results could only have been obtained from a long-term study and emphasize the value of such studies for our understanding of certain aspects of ecology. In addition, they could not have been obtained by a study of the birds alone, being dependent on a knowledge of their insect food; most of this information was made available as a result of a parallel long-term study of the insect fauna of the oak trees, made by a number of people from the Hope Department of Entomology, University of Oxford.

The mechanics of bird migration

Abstract: Summary A theory is presented for calculating the relation between mechanical power required to fly and forward speed, for a bird flying horizontally. The significance of this for migration is explained, and quick methods are given (and summarized in the Appendix) for calculating key points on the curve. Speed ranges and effective lift: drag ratios are calculated for a number of different flying animals. Factors affecting migration range are discussed, and the effects of head- and tailwinds are considered. Still-air range depends on effective lift: drag ratio, but not on size or weight as such. The relation of power required to that available from the muscles is considered. Small birds have a greater margin of power available over power required than large ones, and tend to run their flight muscles at a lower stress, or lower specific shortening, or both. There is an upper limit to the mass of practicable flying birds, represented approximately by the Kori Bustard Ardeotit kori. The effect of adding extra weight (food or fuel) is to increase both minimum-power speed, and maximum-range speed, in proportion to the square root of the weight, and to increase the corresponding powers in proportion to the three-halves power of the weight. Birds up to about 750 g (fat-free) can double their fat-free mass, and still have sufficient power to fly at the maximum-range speed. Larger birds are progressively more severely limited in the maximum loads they can carry, and this reduces their range. Many large birds migrate by thermal soaring, thus economizing on fuel at the expense of making slower progress. During a long flight both speed and power have to be progressively reduced as fuel is used up. A formula is given for calculating the still-air range of a bird which does this in an optimal fashion. The only data required are the effective lift: drag ratio, and the proportion of the take-off mass devoted to fuel. Increase of height has no effect on the still-air range, but the optimum cruising speed (and power) is increased. The optimum cruising height is reached when the bird can absorb oxygen just fast enough to maintain the required power. The optimum height increases progressively as fuel is used up. No range is lost as a result of the work done in climbing to the cruising height.

Re-examination of the capital and income dichotomy in breeding birds

Abstract: During egg-formation, energy and protein are deposited in the developing eggs but are, at the same time, needed by the laying female herself. This has been largely overlooked in the discussion on income and capital breeders (Drent & Daan 1980, Thomas 1988). We discuss data on exogenous versus endogenous energy and nutrients used during egg-formation for 12 well-studied species ranging from the Adelie Penguin Pygoscelis adelie (3400 g) to the Blue Tit Parus caeruleus (11 g) and calculate which part of the total energy and nutrient requirements (of clutch and laying female) originates from direct food intake and/or from body reserves. Because energy and nutrients are also needed by the laying female, some large species breeding in cold regions deposit sufficient reserve that they can fast completely during egg-formation (like the Adelie Penguin) and even throughout incubation (like the Elder Somateria mollissima). However, almost all smaller species must forage for most of their energy and nutrients during the egg-formation period. For the large species, energy and protein of the clutch represent 30% and 70%, respectively, of the total requirements of laying females, much more than in small species like passerines (4% and 40%). Therefore, the requirements for the clutch are much greater in larger than in smaller species, and egg-production is much more limited by protein than by energy. The effects of food supplementation on timing of laying (moderate advance), on number of eggs laid (not more, when corrected for date) and on egg size (not larger) of income and capital breeders/layers are discussed. It seems that neither the start of egg-laying nor the number or quality of eggs are directly related to the energetics of the laying female.

Body size, its heritability and influence on juvenile survival among great tits, parus major

Abstract: Summary This paper reports on part of a three-year study into the biological significance of variation in body size of individuals from a single population of Great Tits in Wytham Woods near Oxford, from 1973 to 1976. It is a reasonable hypothesis that the body size of an individual is controlled partly by heredity and partly by environmental factors. Genetical factors by themselves are shown to account for nearly three-quarters of the total observed phenotypic variance of body size. Of the likely environmental factors considered, only the date of hatching (in 1973) and an annual difference in body size are shown to have significant effects. Previous workers have shown that heavier fledglings have a better chance of surviving the immediate post-fledging period than lighter fledglings (e.g., Perrins 1965). It had been postulated that this differential survival rate might be due to variations in the fat reserves of fledglings. However, the data presented here suggest that this hypothesis is unlikely, and that the body size of fledglings has a more significant influence. The aggressive behaviour of fledgling Great Tits towards one another may be the mechanism responsible for this since it is shown that larger fledglings tend to dominate smaller fledglings.

Factors governing feeding rate, food requirement and brood size of nestling great tits parus major

Abstract: SUMMARY Observations were made on feeding rates and food-consumption of nestling Great Tits Parus major mainly in Larch plantations at lake Yamanaka, Japan. Feeding frequencies were recorded by an automatic recorder. There were marked differences between early and late broods; the feeding frequencies were twice as great in early than in late broods of the same size. No clear tendency was observed in the variations of feeding frequencies in relation to brood size. There was, however, a clear inverse relationship between the frequencies and the average size of food brought to the nests. The males' share in terms of feeding frequencies is described. These figures, however, did not follow the males' contribution in terms of weight of food, which was nearly always higher than the females'. It is pointed out that feeding frequencies are far too variable to be used as a true index of food consumption by nestlings, and are not reliable. Attempts were made to measure the weight of food; the method is described. The average weight of food brought by males was lighter in early than in later broods. The total weight of food was estimated. The trend of daily food consumption per chick was similar to that of the chick's growth curve. It was found that up to about the tenth day of the nestling period daily food-intake per chick increased linearly as body weight increased. At some nests, rate of defaecation was observed. This was at first low, but it increased steeply on the third day, with a steady increase thereafter. By comparing the rates of food intake, faeces output, and weight increment of a chick, it was found that only 20–30% of digested matter (the difference between food-intake and faeces-output was used up daily (for body temperature regulation various external effort, etc.). The factors responsible for this high efficiency of growth in nestlings are discussed. There was a clear inverse relationship between the total weight of food brought per chick per day and the brood size. This is largely because the heat-loss is greater in small than in large broods, so that a chick from a small brood in fact needs more energy to maintain its body temperature after a certain age than one from a large brood. This is discussed in detail. Factors which caused variations in size of food are discussed in relation to feeding frequencies. It is pointed out that, because of the inverse relationship between energy requirement by each chick and brood size, the total food requirement by a brood as a whole did not vary directly in proportion to the brood size. An estimation showed that a b/3 still required about 75% of the total food required by a b/8. A smaller brood is less advantageous than expected to parents feeding nestlings when they encounter adverse conditions, e.g. food shortage in the habitat, or a lack of help by their mates, etc. On the other hand, it is suggested that once they have left the nest, the food-demand by a brood of fledglings the parents have to feed, so that, in the fledging period, in times of food shortage it would certainly be advantageous to have fewer young. It is suggested that, although fledglings may consume three to four times as much food as nestlings, the parents, in providing this food, would not work proportionately harder, since the parents' efficiency of providing food could be higher in feeding the fledglings, which always follow the parents as they are hunting, than in feeding the nestlings to which food has to be brought. On this basis, the adaptive significance of the length of the nestling period in nidicolous species is discussed in relation to clutch size, brood size and food requirement.

Circannual clocks in avian reproduction and migration

Abstract: Many behavioural and physiological functions of organisms are adjusted to the periodic changes in their environment, particularly to those related to the natural day and year. This adjustment is often achieved through the action of endogenous daily (circadian) and annual (circannual) clocks. Studies of the control of avian moult, migration and reproduction have played a major role in understanding how biological clocks function and interact with rhythms in the environment. Investigations on tropical birds such as the East African subspecies of the Stonechat (Saxicola torquata axillaris) and long-distance migrants like the Garden Warbler (Sylvia boriri) have provided the longest records of circannual rhythms, some of them running for more than 12 years, with periods ranging from about 9 to 13 months. Avian circannual rhythms are organized in a characteristic way for a particular species or population, and cross-breeding experiments have shown that some of the differences found among them are genetically determined. In African Stonechats circannual rhythms guarantee that seasonal events occur at the appropriate times of the year and in the characteristic sequence. They also control a “reproductive window” that provides the temporal framework for breeding. The width of this window is rather inflexible but the performance of a bird within this framework (e.g. whether it breeds once or twice per season) is subject to modification by environmental conditions. In migratory birds circannual programs are involved in determining the time course, distance and direction of migration. Circannual rhythms are synchronized with and modified by environmental factors in a complex way, but the endogenous mechanisms usually respond to environmental cues such that an optimal adjustment to season and latitude is guaranteed.

Non‐lethal effects of predation in birds

Abstract: Predators can affect individual fitness and population and community processes through lethal effects (direct consumption or ‘density’ effects), where prey is consumed, or through non-lethal effects (trait-mediated effects or interactions), where behavioural compensation to predation risk occurs, such as animals avoiding areas of high predation risk. Studies of invertebrates, fish and amphibians have shown that non-lethal effects may be larger than lethal effects in determining the behaviour, condition, density and distribution of animals over a range of trophic levels. Although non-lethal effects have been well described in the behavioural ecology of birds (and also mammals) within the context of anti-predation behaviour, their role relative to lethal effects is probably underestimated. Birds show many behavioural and physiological changes to reduce direct mortality from predation and these are likely to have negative effects on other aspects of their fitness and population dynamics, as well as affecting the ecology of their own prey and their predators. As a consequence, the effects of predation in birds are best measured by trade-offs between maximizing instantaneous survival in the presence of predators and acquiring or maintaining resources for long-term survival or reproduction. Because avoiding predation imposes foraging costs, and foraging behaviour is relatively easy to measure in birds, the foraging–predation risk trade-off is probably an effective framework for understanding the importance of non-lethal effects, and so the population and community effects of predation risk in birds and other animals. Using a trade-off approach allows us to predict better how changes in predator density will impact on population and community dynamics, and how animals perceive and respond to predation risk, when non-lethal effects decouple the relationship between predator density and direct mortality rate. The trade-off approach also allows us to identify where predation risk is structuring communities because of avoidance of predators, even when this results in no observable direct mortality rate.

Mass changes in migrating birds: the evidence for fat and protein storage re‐examined

Abstract: The fact that one cannot kill a bird twice makes it very difficult to determine the relative contributions of fat and non-fat components to increases in body mass before migratory flights in individual birds. Knowing the relative contributions of these components is of obvious energetic interest since fat yields about eight times as much energy as fat-free muscle tissue. Several recent studies have failed to demonstrate convincingly, due to flaws in their analyses, that fat-free mass in addition to fat is accumulated before long-distance flights. We point out that regressions of fat or the non-fat component on total body mass cannot yield reliable estimates of the composition of individual mass changes in view of inter-individual variation in structural size, reserve levels or timing of storage. We suggest that studies over time of synchronous populations or marked individuals will give better answers. A re-analysis of published data indicates the widespread existence of fat-free tissue deposition during migration, whereas in some species fat alone explained the increase in total body mass. Larger species tend to incorporate a relatively higher proportion of non-fat components when increasing in mass. However, the comparative data set is not yet of sufficient quality to allow general statements on why, and to what extent in different individuals and species, non-fat tissue in addition to fat is deposited before take-off on migratory flights.

Survival of British Sedge Warblers Acrocephalus schoenobaenus in relation to west African rainfall

Abstract: Fluctuations in the population levels and annual adult survival rates of British Sedge Warblers Acrocephalus schoenobaenus since the late 1960s are strongly correlated with indices of wet season rainfall in the west African winter quarters. Population changes are unrelated to estimates of breeding productivity in the previous year. Habitat availability in the winter quarters has probably been the main factor limiting the size of the Sedge Warbler population in Britain during the period of study.

Sex ratio variation in birds

Abstract: Significant variation in the sex ratio at hatching is unusual in birds In contrast, sex differences in juvenile mortality have been found in a variety of species, especially when food is scarce- In some cases, these differences may be a consequence of reduced viability of males when food is scarce but, in others, the available evidence suggests that parental manipulation is involved

Breeding biology of house martins delichon urbica in relation to aerial insect abundance

Abstract: Summary The breeding biology of House Martins nesting in artificial boxes was examined in relation to aerial insect abundance and weather. The insect food supply was continuously monitored with a suction-trap at 40 ft above ground level. Aerial insect abundance started to rise from the winter level in April and reached a high stable state in June. Insects remained abundant until September; the subsequent decline continued at least until the end of October. The size of first clutches (mean size 3.87) was correlated with the abundance of aphids in spring, though not with temperature. It is suggested that this was an adaptive response because high food levels in May were associated with high levels in the main nesting period. Clutch-size declined through the season but was not matched by changes in food abundance. Second clutches (mean size 2–95) were most frequent following a high food level in June. No correlation was found between egg-weight and preceding food conditions, nor with season, clutch-size or order of laying within clutches. If food was scarce on the day of laying of the first egg, these clutches suffered a suspension of laying of subsequent eggs. Some of the young in these nests showed a reduced growth rate. The duration of incubation (mean 14.6 ± 1.1 days) was not apparently correlated with food abundance, however it must have been potentially extended by extreme shortages because incubation duties were neglected at such times. Infertility was probably the main cause of hatching failure (14.2%). A model was used to predict the date of onset of laying. It was based on the assumption that nestling tissue was produced with the same energetic efficiency as eggs. The model indicated that House Martins could collect enough food to lay eggs earlier than the observed date in most years. The high probability of food shortage before the laying period, however, appeared to discourage laying at this time. The observed onset of laying coincided with the appearance of aphids in the air, probably because they comprised an abundant and stable food source for egg formation. Breeding generally occurred in the period of highest food abundance. Pairs rearing only a single brood each year did so in July when food levels were highest. The growth of first brood nestlings was more closely associated with food levels than any other factor investigated, while in second broods, rainfall was found to have the greatest influence. The total variance in growth explained by environmental factors in the first broods (41%) was greater than in second broods (22%). This relative independence of second broods from adverse environments probably arose from more abundant food and the feeding of some of these broods by the young of first broods. An exceptionally late brood showed an extended nestling period indicating deteriorating conditions later in the breeding season. It is proposed that the spread of laying may reflect differences in the feeding efficiency of nesting House Martins, because the growth of two early first broods was more rapid at given food levels than two late broods. Laying patterns conformed to a distribution based on a progressive threshold mechanism. The more efficient pairs may be able to attain breeding condition and lay at low levels of food abundance, and hence breed earlier in the season. Subsequent layings are facilitated by the rise in food levels in early summer. Mortality within the nest was low (5.8%) and associated with food shortage. Nestling periods (mean 30.6 ± 2.3 days) were not shown to be correlated with food abundance; they were more dependent on brood-size. Recruitment into House Martin populations is thus widely influenced by food supply, particularly through an influence on clutch-size, the occurrence of second clutches and nestling mortality.