Population structure and dynamics in the African Fish Eagle Haliaeetus vocifer (Daudin) at Lake Naivasha, Kenya

Abstract

Summary(1) The population structure and breeding success of the Fish Eagle Haliaeetus vocifer (Daudin) at Lake Naivasha was assessed in 1970‐71, as part of a wider pesticide residue survey. The figures obtained have been combined with those of an earlier population and breeding survey in 1968‐69.(2) Population structure was assessed by plumage criteria, dividing the total adults, subadults (early and late), 2 year‐olds, and recent immatures. Between 1968 and 1971 there was an increase of about 40% in the adult population on the lake, but immatures increased by only 11 %. The adult/immature ratio was 83: 17% in 1968‐69 and 85‐6: 14‐4% in 1970‐71.(3) From these data it appears that the age of Fish Eagles as adults lies between 16 and 24 years, with a total age of 20–28 years. This is much longer than observed wild life spans in some other eagles.(4) Although the total population rose, the number of breeding pairs was constant, fifty‐six in each year of survey, with minor changes.(5) Breeding success overall is about 0–47 young/pair/annum for both years combined, varying greatly between different parts of the lake and from year to year. As only two thirds of all pairs lay, and some of these fail, this equals 0–74 young/pair which bred and 1–28/successful nest.(6) Breeding success at Lake Naivasha is considered to be unaffected by human interference, either deliberate or casual.(7) Analysed by habitat, pairs which bred near wide lagoons within the papyrus were markedly more successful than those with access only to open water (0–55 young/ pair/annum overall cf. 0–29). The possibility that this is due to superior food opportunities in the lagoon habitat is discussed. Fish Eagles can catch birds and fish, and pirate other species kills in lagoons, whereas they must catch all their own fish prey in the open lake sites.(8) Preliminary observations of diurnal behaviour do not indicate that Fish Eagles with access only to open water have any more difficulty in catching prey than those in lagoons, so that the relation between food supply, habitat, and nesting success remains obscure.