The effect of predation by Fish Eagles on the breeding success of various Ciconiiformes nesting near Kisumu, Kenya

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Abstract
The breeding success of a colony of Sacred Ibis and a mixed colony of nine species of Ciconiiformes nesting in trees near Kisumu, Kenya, was studied during 1975. Predation of eggs by Fish Eagles was an important factor contributing to the very low breeding success at both colonies, although clutches laid late in the season were probably abandoned due to low rainfall in April. Only 0·05 chicks fledged per pair at the Sacred Ibis colony, and Yellow-billed Storks were the only species to rear any young to fledging (0·33 per pair) at the mixed colony of Ciconiiformes. Trees in the Sacred Ibis colony were well spaced apart, so that each tree of nesting birds could be regarded as a sub-colony. The percentage egg loss due to predation was inversely correlated with sub-colony size. Fish Eagles did not select small sub-colonies in preference to larger ones, but visited each sub-colony equally. Trees containing many nests therefore effectively swamped the predators with prey, so that the percentage nests visited was inversely correlated with sub-colony size. A similar situation was recorded at the mixed colony. The breadth of eggs laid by Sacred Ibis, Yellow-billed Storks and Black-headed Herons decreased significantly as the season progressed. Eggs laid in the largest Sacred Ibis sub-colony were significantly broader than those from the other sub-colonies and were laid earlier in the season. The possible relationship between sub-colony size and age of the breeding bird were discussed. The decline in the fish stocks of the Winam (Kavirondo) Gulf was mentioned as possibly contributing to the extent of predation by Fish Eagles at these two colonies.