THE NESTING SUCCESS OF THE HERON ARDEA CINEREA IN RELATION TO THE AVAILABILITY OF FOOD

Abstract

SUMMARY The food of nestling herons Ardea cinerea was studied at three heronries in the Thames valley in the years 1952–57. At Buscot and Wytham, Berkshire, fish of the families Cyprinidae and Percidae formed the chief food, but at Buscot many Salmo trutta were also eaten. At High Halstow, Kent, in the Thames estuary, Anguilla anguilla, Scardinius erythrophthalmus, small fish of several species, and shrimps, formed the chief food of the nestlings. It was found that herons caught fish selectively, avoiding both small and large individuals. At Buscot and at High Halstow, but not at Wytham, there were seasonal changes in the proportion of certain prey species which suggest that the heron lays its eggs at such a time that the young are in the nest at the period of maximum availability of food. Young herons hatch asynchronously, and in years of mortality it was always the smallest last‐hatched young that died. It is shown that the adaptive value of asynchronous hatching is that when food is short the smallest young in the nest die, but when food is more plentiful all the young are raised. The average clutch‐size of the heron did not vary significantly, but the survival of young after hatching differed markedly over the six years, and also in the different heronries. These differences were due to starvation, the availability of food to the adult herons being affected by the amount of rainfall and possibly other factors. The percentage of the young surviving was higher in the smaller broods, both in the nest and after fledging.