Population change of avian predators and grey squirrels in England: is there evidence for an impact on avian prey populations?

Abstract

1. Using novel analytical methods applied to extensive national bird and mammal monitoring data, we examine whether 29 English bird populations may have been depressed by increases in the abundance of two broad categories of predators. The first includes predators of juvenile and adult birds: Eurasian sparrowhawks Accipiter nisus, common kestrels Falco tinnunculus and common buzzards Buteo buteo, and the second comprises five nest predators: carrion crow Corvus corone, black-billed magpie Pica pica, Eurasian jay Garrulus glandarius, great spotted woodpecker Dendrocopos major and grey squirrel Sciurus carolinensis. 
2. For 22 avian prey species, there is no evidence that increases in common avian predators and grey squirrels are associated with large-scale depression of prey abundance or population declines. For the remaining seven, some negative correlations are biologically unlikely but we cannot exclude the possibility that some of the negative associations are causally related. For example, a particularly strong negative relationship between sparrowhawk and tree sparrow during the Common Birds Census period (1967-2000) could indicate a causal relationship. In contrast, the negative association between buzzard and goldfinch during the Breeding Bird Survey period (1995-2005) is, on ecological grounds, unlikely to do so. Whilst a correlative study such as this cannot prove causation, it provides a focus for more detailed work on particular species. 
3. Unexpected was a large number of positive associations between predators and prey, particularly for native avian nest predators, which largely exonerates these predators as driving declines in passerine numbers.
4. Synthesis and applications. Analyses of large-scale and extensive national monitoring data provides little underlying evidence for large-scale impacts of widespread avian predators and grey squirrels on avian prey populations, although we cannot exclude the possibility that a small number of negative associations between particular predator and prey species reflect causal relationships or that predators affect prey species at smaller spatial scales.