Modelling large-scale relationships between changes in woodland deer and bird populations.
There is increasing evidence from local studies carried out in several parts of the world to suggest that increases in the abundance of deer may be depressing population levels of breeding woodland bird populations that are associated with dense understorey habitats. We examine whether habitat modification by deer is likely to be a factor contributing to recent large-scale population declines of woodland birds in lowland England. Novel analytical methods are applied to extensive national bird and deer monitoring data, to examine whether populations of 11 woodland bird species that are associated with dense understorey habitats in lowland England may have been depressed following increases in the abundance of three widespread and abundant deer species: Reeves' muntjac Muntiacus reevesi, roe deer Capreolus capreolus and fallow deer Dama dama. An additional four woodland bird species that are not specifically associated with understorey habitats are included as controls. For five of the 11 understorey species considered, there is evidence that increases in deer are associated with large-scale depression of abundance or population declines in lowland England. Of these species, we suggest that the impacts of deer are likely to have been greatest for two species of conservation concern, common nightingale Luscinia megarhynchos and willow tit Poecile montanus. Synthesis and applications. Large-scale analyses of extensive national monitoring data provide evidence for a negative association between three widespread and abundant deer species and several woodland bird populations that are associated with dense understorey habitats. These findings are consistent with those from small-scale experimental studies and indicate that deer-related habitat modification may be affecting some bird species on far larger scales than previously appreciated. Mainly through their effects on understorey vegetation, high deer populations are now likely to be affecting woodland biodiversity over large parts of lowland England and deer management plans, involving integrated exclusion and culling of deer, need to be co-ordinated on large scales. It is suggested that such management plans could most usefully target areas that still support relatively high populations of species that are sensitive to deer. Knowledge about the form of relationships between deer abundance and habitat quality for birds and other biodiversity is an important knowledge gap that needs to be addressed if sound, collaborative deer management plans are to be developed.