The response of a Grey HeronArdea cinereabreeding colony to rapid change in prey species

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Abstract
The fish community of Loch Lomond has changed rapidly in recent years following introductions of 5 new species of fish. The response of the most abundant resident, piscivorous bird, the Grey Heron Ardea cinerea, to the sudden arrival and rapid population expansion of potential prey species is evaluated here. The relative rate of predation of fish by herons during the breeding season and the size of the breeding population before (1978) and after (1990) new species invasions is compared. In addition the relative abundance of fish prey species after the introduction of new species is examined. In 1978 the discarded remains of fish below the nests of breeding herons was dominated by native Roach Rutilus rutilus (63%). By 1990, at the same colony, at the same time of year, discards comprised mainly Ruffe Gymnocephalus cernuus¸ (61%). It would appear that this diet shift was not the result of a lack of available Roach, rather that it was a functional shift in prey choice by herons to a more abundant, more easily caught species. The size of the heron breeding population was slightly smaller in 1990 than in 1978, but increased between 1986 and 1990, although this coincided with a period of rapid growth in the Ruffe population, there is no evidence of a causal relationship.