Supplementary notes on the biology of the large biros of prey of embu district, kenya colony
03 Apr 2008-Ibis (Blackwell Publishing Ltd)-Vol. 97, Iss: 1, pp 38-64
TL;DR: The fluctuations of population in an area of 146 square miles in Embu district, where a census of eagles was carried out in 1950, are described and discussed and the inter-relations of various species are discussed.
Abstract: Summary. 1 The present paper is supplementary to that in ‘Ibis’ 94 and 95. 2 The fluctuations of population in an area of 146 square miles in Embu district, where a census of eagles was carried out in 1950, are described and discussed fur 951–52. 3 The inter-relations of various species are discussed, particularly for Aquila wahlbergi and Lophaetus occipitalis. 4 General accounts of breeding biology are given for Sagittarius serpentarius, Aquila wahlbergi, Hieraaetus ayresi and Terathopius ecaudatus, and supplementary data for Aquila verreauxi, Hieraaetus spilogaster, Polemaetus bellicosus, Stephanoaetus coronatus, Lophaetus occipitalis, Circaetus cinereus and Circaetus pectoralis. These accounts are given under the following heads:— 1 General notes on adults. 2 Nests and nest-building. 3 The incubation period. 4 The fledging period: (a) general, (b) development of the young, (c) parental behaviour, (d) food. 5 The post-fledging period. 5 Special problems of breeding biology are discussed under the following heads: (1) Display; (2) Eagle-weaver-bird nesting-associations; (3) Feeding rates of female and eaglet; (4) Breeding seasons; (5) Breeding success and replacement rate.
TL;DR: In this article, the authors analyzed 105 species of birds of many taxonomic groups from a wide range of geographical localities and found that the shape of the growth curve is not related to the mode of development (i.e. whether precocial or altricial).
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.
TL;DR: It is suggested that Darwinian selection at the level of the individual permits an understanding of the known structure of avian communities and that there is no need at present to invoke new selective mechanisms at thelevel of the community or ecosystem.
Abstract: Territories of birds, usually defended against conspecific individuals, are sometimes defended against individuals of other species. Since such behavior is demanding both of time and energy, natural selection should favor ecological should favor ecological divergence, the establishment of overlapping territories, and the reduction of aggression. Lack of divergence in modes of exploitation could mean that insufficient time has elapsed for the changes to be completed or that the environment imposes some limitation preventing the evolution of the required degree of divergence. Such environmental limitation can be predicted in (a) structurally simple environments, (b) when feeding sites are strongly stratified in structurally complex vegetation, or (c) when the presence of other species in the environment prevents divergence in certain directions. The known cases of interspecific territoriality in birds are analyzed and shown to be largely in accordance with these predictions, although several cases of overlapping territories in situations where interspecific territoriality has been predicted provide relationships worthy of further study. We suggest that Darwinian selection at the level of the individual permits an understanding of the known structure of avian communities and that there is no need at present to invoke new selective mechanisms at the level of the community or ecosystem.
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 22.214.171.124 on Sun, 27 Mar 2016 07:03:43 UTC All use subject to http://about.jstor.org/terms 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 126.96.36.199 on Sun, 27 Mar 2016 07:03:43 UTC All use subject to http://about.jstor.org/terms October 1984] Care and Feeding of Golden Eagles 755
TL;DR: The continuation of work on eagles in Embu district, Kenya, especially at Eagle Hill, which has now been under observation continuously since 1949, is described and possible life spans in the wild state of adults of H. dubius and S. coronatus are suggested.
Abstract: SUMMARY This paper describes the continuation of work on eagles in Embu district, Kenya, especially at Eagle Hill, which has now been under observation continuously since 1949. Observations in other parts of Kenya have been included. The ecological changes possibly affecting eagles on Eagle Hill are discussed. The population fell from a pair each of Circaetus cinereus, Aquila verreauxi, Hieraetus fasciatus spilogaster, H. dubius, Polemaetus bellicosus and Stephanoaetus coronatus in 1952 to a pair each of H. dubius, P. bellicosus and S. coronatus in 1965. Possible causes of the decline are discussed. The species of eagles are not normally aggressive to one another, in contrast to other resident species such as Falco peregrinus and Buteo rufofuscus. Although the eagles appear to be ecologically separated by food preferences and habitat this is apparently not the whole explanation for the unusual concentration of eagles on this hill. Additional breeding data are given for H.f. spilogaster, H. dubius, P. bellicosus and S. coronatus. These species rear respectively 0.56, 0.65, 0.42 and 0.44 young per pair per annum. S. coronatus breeds in alternate years and cannot breed every year because of a protracted post-fledging period in which the young is fed for up to 350 days. P. bellicosus, with about the same annual reproductive rate, does not have the same breeding rhythm. Data on reproductive rates combined with other data suggest possible life spans in the wild state of adults of H.f. spilogaster 10–11 years, H. dubius nine years, P. bellicosus 14 years, and S. coronatus 16 years. At nests of H. dubius and S. coronatus changes of mates have been recorded for 16 and 17 years respectively. In S. coronatus a change occurs about every six years and in H. dubius about every four years, indicating that S. coronatus may live about 1.5 times as long as H. dubius in the wild state. One female S. coronatus was known to live for 8.5 years as an adult. Other incomplete life spans are eight and eight years for two male S. coronatus, and eight for one female of this species. Two male H. dubius have each lived for at least eight years but no female of this species has lived for more than five years. Two proven cases of re-laying after a natural disaster are recorded, one each in H. dubius and S. coronatus. Other instances are suspected in H. dubius. The habit may be commoner than is supposed in large eagles. The history of four pairs of S. coronatus, each observed for four years or more, totalling 34 pair/ years is given. S. coronatus breeds regularly every second year unless some unusual occurrence, such as a change of mates or a failure during incubation, upsets the rhythm. S. coronatus females lay 1–2 eggs at dates varying from June–October in Kenya; breeding is not confined to the dry season. Laying dates of individual females may vary by two months between one year and another. Incubation takes 48–49 days, fledging 105–116 days. The elder of two young hatched invariably kills the younger so that no more than one young is reared. Female adults are dangerously aggressive, especially during days 30–60 of the fledging period. In 86% of cases where eggs are laid a young bird is reared. Since clutches of two in practice do not result in more than one young this represents a breeding success of 86% of the potential, a very high percentage. The sex ratio of young leaving the nest is about equal, seven males to five females, in known cases. The post-fledging period in S. coronatus is 330–350 days, and the total breeding cycle about 560 days, making it impossible for the eagles to breed every year, if they rear a young bird to independence. In the post-fledging period the young S. coronatus remains within half a mile of the nest, where it is fed by the parents, the female bringing most of the prey. The adults call to attract the young bird, which flies into the nest receiving the prey there, or rarely on a tree nearby. If the adult obtains no response from the young it may carry the prey away. Although regularly fed by its parents the young eagle kills some of its own food from at least day 61 of the period onwards, but most often in the last third of the period, being then apparently stimulated by unusual periods of privation. Almost 100% of young eagles that leave the nest are reared to independence at about 15 months old. The possible biological advantages of this protracted adolescence in survival and economy of prey are discussed. The main prey of S. coronatus is antelopes, followed by hyrax. Monkeys are rarely taken. Killing methods, times, and relations with prey are discussed. The eagles usually kill in early morning or evening, but also at other times. They may cache portions of large kills. Most prey is brought to the nest between hours 4–6 of daylight. The male S. coronatus feeds his incubating mate about once every 3–3 days. Once the young has hatched his killing rate rises to about one kill per 1.7 days. The killing rate falls slowly to one kill per two days later in the fledging period. At normal times the killing rate of adults is apparently controlled by their own appetites, and the increased killing rate of the male after hatching is an exception to this rule. During the post-fledging period the feeding rate varies from 1: 2.0 days to 1: 6.2 days, averaging 1: 3 days in 130 cases. Periods of privation may last from 5–13 days. Alternatively several kills may be brought in a day, possibly from cached portions of large kills in some cases. Long foodless periods may stimulate the young eagle to kill for itself, especially in the last third of the post-fledging period. Final independence of the young is not brought about by aggressive parental behaviour, but is probably due to increasing indifference of the young to food-bringing adults. This indifference may act as a release to the adults, breaking the rhythm of bringing food to the young, and so stimulate the onset of a new breeding cycle.