Evidence for bottom-up effects of moth abundance on forest birds in the north-boreal zone alone.

Insect declines are raising alarms regarding cascading effects on ecosystems, especially as many insectivorous bird populations are also declining. Here, we leveraged long-term monitoring datasets across Finland to investigate trophic dynamics between functional groups of moths and birds in forested habitats. We reveal a positive association between the biomass of adult- or egg-overwintering moths and the biomasses of resident and long-distance migrant birds reliant on caterpillars as breeding-season food in the north-boreal zone. Contrary to expectations, similar signs of moth bottom-up effects on insectivorous birds were not observed in other Finnish regions or for moths overwintering in other life stages. In fact, some negative associations between moths and birds were even detected, possibly attributable to opposite abundance trends. While supporting the existence of bottom-up effects in the north-boreal zone, our study emphasizes the need for further investigation to elucidate moth-mediated trophic dynamics in areas characterized by the insect decline.