OBSERVATIONS ON SOME KENYA EAGLES
Authors: Brown L. H. 
Published: April 2008

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.