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Journal ArticleDOI

Observations on some kenya eagles

28 Jun 2008-Ibis (Blackwell Publishing Ltd)-Vol. 108, Iss: 4, pp 531-572
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.
Citations
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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
Adrian Treves1
TL;DR: It is concluded that predation is not a uniform selective pressure and patterns of social behavior within groups do not predict antipredator behavior.
Abstract: I studied antipredator behavior in two species of monkeys to elucidate the role of predation in shaping the social systems of arboreal primates. I compared the responses of monkeys to auditory and visual contact with predators to response elicited by sound playback experiments using the recorded calls of predators. Changes in vigilance and aggregation persisting up to 30 min after predator encounter occurred in both cases. Measures of vigilance shed light on individual perceptions of risk, while aggregation measures—intragroup spatial cohesion and polyspecific associations—permit direct inference about the protective benefits of grouping for the monkeys. They responded to real predator encounters and simulations in similar ways. Thus, sound playbacks of predator vocalizations are effective to simulate predator proximity. Contrary to predictions, predator encounters did not lead invariably to increased cohesion within groups or to increased time spent vigilant. Moreover, behavior in polyspecific associations was no different from that in single-species groups. Only red colobus encountering chimpanzees behaved as predicted by increasing vigilance and intragroup cohesion. The red colobus social system may have developed to protect against chimpanzee attack. In contrast, red-tailed monkey encounters with raptors and chimpanzees involved no change in time spent vigilant, coupled with decreases in intragroup cohesion. I conclude that predation is not a uniform selective pressure and patterns of social behavior within groups do not predict antipredator behavior.

119 citations

References
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Journal ArticleDOI
03 Apr 2008-Ibis
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.

39 citations

Journal ArticleDOI
03 Apr 2008-Ibis
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.

29 citations

Journal ArticleDOI
28 Jun 2008-Ibis
TL;DR: The data on food indicate that the African Fish Kagle eats more birds than has generally been supposed, especially Hamingos; the diet may be summed up as fish when abundant, otherwise water-birds and some carrion.
Abstract: SUMMARY 1. The African Fish Eagle Haliaetus vocifer is discussed, with especial reference to the Kavirondo Gulf area of Lake Victoria. 2. Numbers in I his and adjacent areas are given, and reference is made to possible movements on Lake Victoria. 3. Territories and home ranges of seven pairs breeding on a group of islands overlap somewhat, with a tendency towards mutual stimulation by adjacent pairs, and the possibility of breeding and hunting difficulties through overcrowding. 4. Some pairs appear to be permanently resident in certain areas without breeding at all. 5. The data on food indicate that the African Fish Kagle eats more birds than has generally been supposed, especially Hamingos; the diet may be summed up as fish when abundant, otherwise water-birds and some carrion. 6. The breeding cycle is discussed under the heads Display, Nests and Nest-building, Eggs and Incubation, the Fledging period, and the Post-Hedging period. 7. Breeding seasons, and their possible relations to food supplies and climate are discussed. 8. Three years' data on breeding success indicate a mean annual replacement rate in the Kavirondo Gulf of 0–6 young per pair per annum, with wide variations in the performance of individual pairs.

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Journal ArticleDOI
03 Apr 2008-Ibis
TL;DR: The range of several species is extended by new Nigerian records and the distribution of the races of other species more clearly defined.
Abstract: Summary A systematic account (with field-notes) of the birds of south-eastern Nigeria is given, based on a collection of about 350 skins made between 1946 and 1949, mainly in the neighbourhood of Owerri The range of several species is extended by new Nigerian records and the distribution of the races of other species more clearly defined

14 citations