Avian nutritional condition increases with forest structural complexity

Abstract

Sylvicultural practices are increasingly directed at forest conservation, restoration and sustainable management. Forests with higher structural complexity are expected to be more resilient against climate change impacts and to have a higher diversity of ecological niches. The impact of forest structural complexity on individual organisms, however, is less well known. To bridge this knowledge gap, we here test if structurally more complex forests provide a more favourable nutritional environment for forests birds, using ptilochronology as an indicator for individual condition. We therefore measured the width of growth bars in tail feathers of great tits (Parus major) captured in 19 forest plots along a gradient of increasing structural complexity. After validating our assumption that series of growth bars reflect the outcome of an energetic income vs. expenditure balance with possible consequences for health and fitness, we show that birds in structurally more complex forests maintain a better nutritional status, probably due to a higher and more stable availability of food resources and more sheltered habitat. Dominant tree species, architecture and associated ecology also play an important role, as some of the observed relationships appear to be tree species-specific. Overall, we demonstrate the importance of a complex forest structure and herb layer, and argue for the need for individual research on the condition, performance and health of forest species to inform managers and policymakers on the impact of changing forestry practices on biodiversity and ecosystem functioning.