Links between an insectivorous bird and moths disentangled through national scale monitoring data

Insects play important roles in food chains, but quantifying how insect abundance affects population dynamics in natural communities is challenging. National scale monitoring data provides opportunities to identify trophic relationships at broad spatial and temporal scales but requires careful approaches to link data from different schemes. Here, using two monitoring datasets from Great Britain, we apply a two-step process to reveal how the population dynamics of the blue tit Cyanistes caeruleus is influenced by the abundance of moths - a core component of their breeding diet. We first find that at a national scale, years with increased population growth for blue tits strongly correlate with high average moth abundance, but population growth in moths and birds is less correlated; suggesting moth abundance affects bird population change rather than shared responses to environmental variation. Next, we identify moth species that are important components of the blue tits' diet, recovering associations to species previously identified as key food sources such as the winter moth Operoptera brumata. Our work provides rare evidence that insect abundance can impact bird population dynamics in natural communities and provides insight difficult to obtain from smaller-scale observations as we evaluate spatial diet turnover at a national scale.