Population collapse of a common forest passerine in northern Europe as a consequence of habitat loss and decreased adult survival

Understanding how anthropogenetic change affects populations of species is crucial to halt the loss of biodiversity. Although habitat loss and degradation are key drivers of population declines, the exact demographic mechanisms are seldom well understood. Here, we investigated how habitat availability and changes in habitat availability were related to large-scale changes in the breeding and winter populations of a rapidly declining forest species, the willow tit Poecile montanus, in Northern Europe. In addition, we examined the effects of potential competitors (the great tit Parus major and the blue tit Cyanistes caeruleus) and their predator (the great spotted woodpecker Dendrocopos major) on long-term population changes. We also investigated long-term trends in adult survival and recruitment. Changes in breeding numbers were positively associated with the availability of older forest and changes in great tit numbers, but not significantly associated to short-term changes in forest cover or the abundance of nest predator, great spotted woodpeckers. Site level population changes during winter – a proxy for survival rates – were positively associated with the amount of older forest, but were not affected by temperature. The capture-recapture study showed a strong decline in adult survival, but the per capita recruitment rate showed no clear trend. As adult survival has a large contribution to the population growth rate, our results suggest that the willow tit population decline is mainly caused by decreased adult survival rates during non-breeding season. Given that logging volumes have been increasing in recent decades in the area, the reduced habitat quality and loss of older forests are likely the key drivers of the overall population decline.