Responses of woodland birds to increasing numbers of deer: a review of evidence and mechanisms

Increases in grazing pressure within woodland can profoundly affect the species composition of bird communities, but few studies have been conducted in Europe and there is a lack of relevant experimental research. A principal mechanism by which deer can alter bird communities is through reduction of low foliage density. High deer densities may also be implicated in a variety of indirect effects on birds mainly by altering food resources and nest losses through predation. Breeding populations of migrant birds appear to be especially vulnerable to increased deer densities in lowland British woods because these species generally require dense understorey structures. Evidence is presented that, possibly with the exception of nightingale ( Luscinia megarhynchos ), populations of breeding migrants may persist in lowland woods with increasing deer densities, provided that young regeneration is adequately protected. However, a priority for research is to understand how bird communities respond to different densities of deer.