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Journal ArticleDOI

On the biology of the large birds of prey of the embu district, kenya colony.

03 Apr 2008-Ibis (Blackwell Publishing Ltd)-Vol. 95, Iss: 1, pp 74-114
TL;DR: The species of eagles occurring in Embu district are detailed, with general notes on the methods and scope of the study, and the inter-relations of the various species are discussed from the points of view of territorial agressiveness and competition for prey.
Abstract: SUMMARY. 1 The species of eagles occurring in Embu district are detailed, with general notes on the methods and scope of the study. 2 The vegetation and climate of Embu district are described, and their effect upon eagles is discussed. 3 Population and inter-relations. The actual population of eagles and of the Secretary Bird in an area of approximately 146 sq. miles is given and their remarkable local concentrations are described. The inter-relations of the various species are discussed from the points of view of territorial agressiveness and competition for prey. 4 Detailed accounts are given of the breeding biology of Sagittarius serpentarius, Aquila verreauxi, A. wahlbergi, Hieraaetus spilogaster, Polmaetus bellicosus, Stephano-aetus coronatus and Circaetus cinereus, and some information for Terathopius ecaudatus, Circaetus pectoralis, and Aquila rapax, Hieraaetus ayresi, Lophaetus occipitalis, and Cuncuma vocifer. The headings for each species include: “Nests and nest-building”, “Incubation period”, “Fledging period” (with special attention to food), and “Post-fledging period”. 5 The following special aspects of breeding behaviour are described and discussed: (1) Display, (2) Use of green branches, (3) Breeding seasons, (4) Breeding success, (5) Breeding frequency.
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TL;DR: How cainism might in fact be adaptive for the parents as well as for the older chick is discussed, and it is demonstrated that cainsism can be selected for even when it decreases the reproductive productivity of the adults.
Abstract: Many nestling hawks and owls die before fledging. Incubation of eggs usually begins with the first egg and because eggs are laid singly at intervals of several days, the young usually hatch at similar intervals. First-hatched birds often receive an advantage of greater food intake over younger siblings. During years of low food abundance, the younger birds may starve, and dead young are occasionally fed to (or eaten by) their siblings. Older chicks may even be responsible for the final demise of the starving chick (e.g., Pilz and Siebert, 1978). Lack (1966) has argued that this reproductive pattern maximizes the reproductive output of the parents. In years of high food abundance, the adults are able to feed all of the young adequately; in other years, wastage of food on young destined to die is minimized, thereby maximizing the reproductive success of the parents. Such a mechanism seems to pertain to tawny owls (Strix aluco: Southern, 1970), goshawks (Accipiter gentilis: Schnell, 1958), and hen harriers (Circus cyaneus: Watson, 1977) among others. Another behavior, which is more problematical for the evolutionary biologist, has been documented for other raptors such as lesser spotted eagles (Aquila pomarina: Meyburg, 1974) and black eagles (A. verreauxi: Gargett, 1978). These birds also produce more young than they usually fledge, but instead of younger chicks passively starving to death, the older chick attacks and can cause the death of its sibling(s) shortly after they hatch, often regardless of immediate food abundance (Gargett, 1970a; Meyburg, 1974). The older chick may not deliver the coup de grace to the younger chick, but nevertheless the demise of the younger is due to harassment and intimidation from the older chick (e.g., Rowe, 1947; Meyburg, 1974; Gargett, 1978). The adaptive basis of such fratricidal behavior, often referred to as cainism (e.g., Brown, 1976), is unclear (Brown, 1955, 1970, 1976; Ingram, 1959; Brown and Amadon, 1968). While starvation of chicks by siblings who dominate the food supply allows the survival of younger chicks during years of abundant food, active destruction of the younger chicks shortly after hatching never permits their survival. Seemingly, cainism should result in a lower average reproductive output than starvation, and consequently should be selected against. In this note, I discuss how cainism might in fact be adaptive for the parents as well as for the older chick, but I also demonstrate that cainism can be selected for even when it decreases the reproductive productivity of the adults.

134 citations

Journal ArticleDOI
01 Jul 1983-The Auk
TL;DR: Le comportement fratricide chez les Accipitridae apparait lie a des differences relatives de taille au sein of the fratrie, differences elles-memes liees au temps ecoule entre les eclosions.
Abstract: Le comportement fratricide chez les Accipitridae apparait lie a des differences relatives de taille au sein de la fratrie, differences elles-memes liees au temps ecoule entre les eclosions, aux differences de poids a l'eclosion, peut-etre au sexe

113 citations

Journal ArticleDOI
01 Oct 1984-The Auk
TL;DR: The division of labor between the sexes of Golden Eagles during breeding is quantified and these activities to the food consumption of nestlings are related to theories of sexual size dimorphism and parental investment.
Abstract: -A field study of Golden Eagles (Aquila chrysaetos) nesting in and near the Snake River Birds of Prey Area was conducted during 1977-1979. Patterns of parental care differed between female and male eagles during incubation and chick rearing; males consistently captured more food throughout all phases of brood rearing (1.2 vs. 0.6 prey/day), while females typically fed and tended the offspring. During the 7th through 9th week of chick rearing, when the food requirements of nestlings were greatest, the female contributed 43% of the prey biomass. No differences were observed in mean daily capture rates between 1978 and 1979 or between parents of one-chick broods and parents of two-chick broods. Although there were no differences between the sexes in the mean weight of prey captured, there were significant differences among pairs, suggesting differences in prey availability or hunting ability. The daily food consumption of eaglets increased as chick rearing progressed and peaked between the 7th and 9th week. Comparisons between eaglets in different-sized broods revealed that individuals in multiple-chick broods received more food from adults than those in one-chick broods. Late in chick rearing, however, those chicks competing with siblings for food had lower consumption rates. Received 24 February 1984, accepted 1 May 1984. THE general nesting biology of Golden Eagles (Aquila chrysaetos) has been described by many naturalists (e.g. MacPherson 1909, Gordon 1927, Bent 1937). Several studies also have been conducted specifically on territory size (Dixon 1937), molt (Jollie 1947), and growth (Sumner 1929, 1933). More recently, research on Golden Eagles has focused on diet and food requirements (e.g. Fevold and Craighead 1958, McGahan 1967, Mollhagen et al. 1972) and nesting success (e.g. Smith and Murphy 1973, U.S.D.I. 1979). Although these studies contributed greatly to our understanding of eagle biology, none has described the relationship between nestling food consumption and parental care. In this paper, I quantify the division of labor between the sexes of Golden Eagles during breeding and relate these activities to the food consumption of nestlings. The size and total biomass of prey delivered to young by male and female eagles also are considered in relation to theories of sexual size dimorphism and parental investment. STUDY AREA AND METHODS The study was conducted along the Snake River Canyon and surrounding upland desert plateau south ' Present address: School of Forest Resources and Conservation, 118 Newins-Ziegler Hall, University of Florida, Gainesville, Florida 32611 USA. of Boise, Idaho. This 195,063-ha area, known as the Snake River Birds of Prey Area (BPA), is administered by the Bureau of Land Management and lies within the Great Basin semidesert scrub biome (Whittaker 1975). The major vegetation types in the area include big sagebrush (Artemisia tridentata) associations, grasses (Poa and Bromus spp.), and shadscale (Atriplex confertifolia). Approximately one-fifth of the BPA is cultivated. A more detailed description of the vegetation can be found in U.S.D.I. (1979) and Collopy (1980). Incubation data were collected in 1977-1979 from 11 nesting attempts. Weekly observations at each site were made from a prominent location 150-750 m from the nest, and the amounts of time each parent spent incubating or brooding were recorded. Instances of male eagles providing prey to females when relieving them from incubation also were recorded. Data during the nestling period were collected at the same four nest sites in 1978 and in 1979. Daylong observations at each study site were made once every 6 days from blinds 15-40 m away. Photographs showing unique plumage characteristics of the breeding adults in 1978 and in 1979 revealed that the same individuals nested at the same sites in both years. The sex of parents was determined from these photographs, from size differences, and from behavior. I identified parents during each nest visit by using these unique plumage characteristics and by comparing photographs of adults taken during each visit. Adults away from the nest were monitored by a second observer, so when identification of the parent on the nest seemed uncertain it was confirmed by accounting for the location and sex of its mate. For a detailed description of nestling diet and nest 753 The Auk 101: 753-760. October 1984 This content downloaded from on Sun, 27 Mar 2016 07:03:43 UTC All use subject to 754 MICHAEL W. COLLOPY [Auk, Vol. 101 observation and visitation procedures see Collopy (1983a). Parental care of nestlings involved both sheltering and feeding. Sheltering activities included brooding and shading, and both are discussed in this paper. Both the delivery of prey to the nest and its consumption by nestlings were considered feeding activities. The parental care of each adult was analyzed in relation to the age of its offspring. Following each observation period, I measured the body weight and foot-pad size (tip of hallux to tip of middle toe on extended foot) of the chicks (Kochert 1972). Determination of the sex of each chick was made late in the nestling period when size dimorphism became obvious. All prey delivered to the nest during each observation period were identified to species and assigned to a size class. The estimated proportion of the carcass delivered and sex of the eagle delivering the prey also were recorded. I calculated prey biomass delivered to nests from the estimate of the proportion of the carcasses delivered and the species' weights (Steenhof 1983). A series of experiments on the food consumption and growth energetics of captive Golden Eagle chicks was conducted concurrently with this study (Collopy 1980). These feeding trials were designed to monitor the consumption rates of eaglets presented blacktailed jackrabbit (Lepus californicus) food ad libitum and to quantify their growth rates. Because of permit restrictions, the birds were tested only between the ages of 11 and 57 days old. Following the experiments, they were returned to foster eagle nests in the wild, from which they all successfully fledged. During the feeding trials, it was apparent that one meal each day was much larger than all others and that it represented the maximum quantity a chick that age could consume. I quantified this relationship for the two female and two male eaglets tested by expressing the maximum meal size (Y, grams) as a function of age (X, days): female: Y = -99.96 + 12.31X; R2= 0.87, P < 0.0001; male: Y = -20.76 + 7.68X; R2= 0.85, P < 0.0001. Following each meal, the percentage of the crop of each wild nestling that was full was estimated, and the amount of food consumed was calculated. Statistical procedures used to analyze data included the Chi-square test, two-sample t-test, and analysis of variance (Remington and Schork 1970). Assumptions of the normality and equal variance of the statistical models were tested; percentage data were arcsine transformed before analysis whenever they were outside the interval between 30 and 70%. All means are reported with standard errors. RESULTS Incubation.-A total of 692 daylight hours (56 observation days) of data was collected at 11 Golden Eagle nests during incubation in 19771979. At the 10 sites that hatched young, female eagles spent a significantly greater portion of the day incubating (82.6 ? 1.6%) than males did (13.8 ? 1.8%) (t = -22.90, P < 0.0001). Eggs were left exposed only 3.7 ? 0.4% of the daylight hours. In addition to performing the majority of the daytime incubation, only females incubated at night. Overall, males relieved incubating females 2.1 ? 0.1 times daily and averaged 49.4 ? 4.7 min per incubation bout. Of the 111 male-initiated changeovers, 17 (15.3%) involved food transfers to the female on or near the nest. Eagle behavior away from the nest was not monitored systematically during incubation; females occasionally were observed foraging on their own, however, when males did not provide them with food. The unsuccessful eagle pair abandoned their nesting effort during the third week of incubation in 1978. The male incubated only once during my 23.4 h of daylight observations and did not deliver any food to his mate. The lower incubation time of the female (67.5% of daylight hours) and the greater exposure time of the eggs (31.6%) suggest that inattentiveness by the male may have forced the female off the nest to forage and ultimately to abandon her effort altogether. No direct evidence exists that the male who successfully bred at this site in 1977 died or was supplanted, but the lack of synchrony between the pair in 1978 suggests that a different male was present. Brooding/shading nestlings.-A total of 1,248 daylight hours (86 observation days) of data was collected during chick rearing at eight nests in 1978-1979. Chick rearing was defined as the period between the hatching of the first egg and the fledging of the last offspring. Although males regularly landed on nests to deliver prey, they were present only 0.6 ? 0.2% of the observation time. I observed a male brooding and feeding nestlings only once during the study. Clearly, the parental role of males during brood rearing was to provide food, because essentially no time was invested in brooding or feeding young. Several other workers who closely monitored parental behavior at the nest also reported that male eagles rarely brooded or fed young (Hunsicker 1972, Hoechlin 1974, Ellis 1979). This content downloaded from on Sun, 27 Mar 2016 07:03:43 UTC All use subject to October 1984] Care and Feeding of Golden Eagles 755

89 citations

Journal ArticleDOI
TL;DR: It is hypothesized that an apparent very low density of reptiles in a wide variety of African habitats is due to exceptional predation pressure on reptiles by a large array of carnivores that are maintained in two ways by the exceptionally large biomass of large herbivores in these habitats.
Abstract: I hypothesize that a large biomass of large wild herbivores in a habitat should severely depress the biomass of reptiles in that habitat. This is based on two somewhat distinct kinds of predation on reptiles by carnivorous birds and mammals, and, more indirectly, on two kinds of habitat modification by large herbivores. Habitat modification will receive little attention here because there is virtually no information on it in the current literature. Below, I first deal briefly with the hypothesized patterns and processes and then examine the African predator-prey literature for natural history information which suggests that these hypotheses are reasonable. I have been unable to locate any data to test directly the hypotheses that reptile density is lowered in the presence of large herbivores or that, if it is, the large herbivores are the cause. These hypotheses were prompted by the observation that, to me, Kenya, Uganda, and Cameroun reptile biomass appears much lower than that in comparable habitats in the neotropics.

66 citations

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Journal ArticleDOI
03 Apr 2008-Ibis
TL;DR: Proximate factors provide a physiological timing mechanism whereby the gonads recrudesce, and eggs are laid, at such a time that, on the average, the young hatch at a season when there is sufficient food to raise them.
Abstract: Summary. 1 In northern Europe, the northward movement of the sun in spring has so extensive an influence, that the food of all species of birds reaches its maximum at some time between late spring and early autumn. As a result, and in contrast to the tropics, nearly all species of birds lay their eggs in roughly the same period of the year. 2 A review of the breeding seasons of British birds reveals some interesting correlations between time of breeding and time of maximum food, and also many apparent puzzles, particularly as regards the marked differences in breeding season sometimes found between related species. 3 In Lapland, all species start breeding 1–2 months later than in Britain, and the breeding season is much less extended. As compared with Britain, the Corvidae and predatory birds of Lapland lay early relative to the small passerines, but among the small passerines, and also among the predators, the different species tend to breed in the same order relative to each other as they do in Britain. 4 Quantitative measurement of breeding seasons is much needed, and some examples showing the advantages of this method are given in Tables 1–3 and text-Figs. 1–5. 5 It is considered that, through the action of natural selection, the breeding season of each species coincides with the time of year when offspring can be raised with greatest success. The major ultimate factor involved is, therefore, the food for the young birds (and hence the factors which determine the abundance and availability of this food). But there may be modifying ultimate factors concerned with the survival of the parents, of the nest and eggs, and of the juvenile birds in the period soon after they leave the nest. 6 Proximate factors provide a physiological timing mechanism whereby the gonads recrudesce, and eggs are laid, at such a time that, on the average, the young hatch at a season when there is sufficient food to raise them. In European birds, daylength seems the major proximate factor, but there are several modifying factors, perhaps including temperature.

213 citations

Journal ArticleDOI
03 Apr 2008-Ibis
TL;DR: The observed timing of breeding seasons can be secured only by external factors regulating an internal rhythm, and in some communities and categories of birds the “reasons” for the observed breeding seasons are intelligible.
Abstract: Summary. 1 For the present purpose Africa is divided into everyreen, semi-arid and intermediate (deciduous) types of count?, all of which occur in all latitudes south of the Sahara. 2 “Breeding season” is limited to the months during which eggs are laid by the species concerned in the given area and all records are interpreted accordingly. As a basis for ascertaining the curve of breeding activity in a given area through the year the number of species laying in each month has been ascertained and calculated as a percentage of the total for all species for the whole year. 3 A definite peak in the curve of breeding activity is evident everywhere except in certain areas within about four degrees of the Equator. 4 In one part of this inner tropical belt there may be no distinct breeding season for must groups of birds (Congo), but in East Africa a double breeding season is the rule, with peaks coinciding with the two rainy seasons. 5 Even so close to the Equator as 5°s. the (single) breeding season in evergreen forest is as restricted as in other types of country and its time-relation to the rains varies locally. 6 In the “ intermediate” type of country characterized by 4–6 months drought each year the timing of the peak breeding season varies from the end of the rains, at Cape Town, to the start of the rains in Natal and several weeks before the rains in areas 23° - 10° S. 7 The key to this local difference is that at Cape Town the rains fall in the cold season, so that vegetation and insects are slow to flush. In the warmer conditions in which the rains begin in Natal the flush comes at once. And further north the dominant vegetation and its associated insects flush towards the end of the drought and well in advance of the rains. 8 From Natal northwards the breeding season for all birds combined shows a progressively less marked peak. The reason is that the seasons of certain ecological categories (1) water birds, (2) raptors and scavengers, (3) ground birds, (4) grass birds, (5) the other birds, tend to diverge. 9 The raptors and scavengers are everywhere the earliest breeders, the biggest species laying by the middle of the dry season. The water birds lay to a large extent towards the end of the rain, and after. The ground birds tend to lay as soon as the grass fires are over and before the heavy rains have induced a lush growth of herbage. The grass birds lay later than most-others, when the grass has grown high. 10 In semi-arid areas the breeding seasons are on the whole similar to the foregoing, with most birds breeding when the vegetation flushes, whether just before or after rain has fallen. But the “semi-and” birds are notably sensitive to rainfall; breeding that has begun is checked if the rains are interrupted. 11 In some communities and categories of birds the “reasons” for the observed breeding seasons are intelligible, the best food-supply or the safest nesting apparently being secured. In others the reasons are not obvious; and the degree to which the breeding seasons are restricted is often incomprehensible. 12 The observed timing of breeding seasons can be secured only by external factors regulating an internal rhythm. Day-length, rainfall and humidity, temperature and visual stimuli are each considered briefly. Each may be effective on some species in some areas, but no one generally.

123 citations

Journal ArticleDOI

115 citations