The diet of Grey HeronsArdea cinereabreeding at Loch Leven, Scotland, and the importance of their predation on ducklings

Abstract

Regurgitations from nestling Grey HeronsArdea cinereaat Loch Leven, Scotland, April‐July 1981–83, contained mainly three types of prey:perch, brown trout and ducklings. The diet varied markedly through the season but not between years. As the heron breeding season progressed, perch occurred in fewer regurgitations and ducklings in more. There was no seasonal variation in the occurrence of brown trout. Regular collections of duckling down from the heronry suggested that the consumption of ducklings peaked in early June. The ducklings taken by herons were less than 10 days old, Mallard ducklings predominating before mid June and Tufted ducklings later. Most ducklings were taken by only a few herons:those that bred earliest and which initially fed their young on perch. Ducklings became a major part of their diet in the late nestling period and broods fed on ducklings fledged no more or fewer young than others. Herons feeding their young on brown trout took ducklings rarely, if ever. It is argued that variation in the contents of regurgitations resulted from three interacting variables; the type of feeding habitat used by individual Grey Herons, the date at which they bred and the date that regurgitations were produced by their nestlings. The numbers of ducklings taken by herons were calculated to be about 230 in 1981 and 291 in 1982, about 5% of Mallard and 3:b of Tufted ducklings estimated to have hatched annually. Assuming herons continued to consume ducklings at the same rate after their young had dispersed, the figures for Tufted ducklings would be higher but still only about 4% in 1981 and 6% in 1982. Compared with total duckling losses of over 75%, predation by Grey Herons was minor and did not affect duckling production in the years concerned.