Possible roles of habitat, competition and avian nest predation in the decline of the Willow Tit Parus montanus in Britain

Abstract

Capsule: Although stable in their preferred, wet habitats, Willow Tits have declined in woodland, probably because of habitat degradation rather than increases in avian nest predator and competitor numbers. Aims: To elucidate the cause of the population decline by investigating variations with habitat and testing the hypothesis that an increase in nest predators or competitors (especially for nest-holes) could be responsible. Methods: I analysed with respect to survey plot habitat over 30 years of Willow Tit abundance data from a national survey. I examined the relationships with the local abundance of key potential nest predators (woodpeckers and Jay Garrulus glandarius) and key potential competitors (other tit species and Nuthatch Sitta europaea). Results: Willow Tits have declined in farmland and woodland in Britain, but not in their preferred, wet habitats. There was no evidence for any negative effect of competition and the only potentially important negative correlation with the abundance of a predator (Great Spotted Woodpecker Dendrocopus major) was limited to farmland, a relatively minor habitat. A negative relationship with Green Woodpecker Picus viridis probably reflects opposite habitat preferences. Conclusions: Although correlative studies of competition and predation cannot be definitive, the evidence of this study suggests that the major cause of the Willow Tit's decline in Britain is habitat degradation, especially in woodland. Possible mechanisms for this degradation are discussed.