Resource declines shape phenological and morphological responses to climate change

Biodiversity is declining, with cascading effects of defaunation expected across trophic levels. Widespread population declines may drive general biotic responses to global change and determine their fitness effects. We find that a 62% decrease in insect biomass over a half-century altered the morphology, survival, and breeding phenology of an aerial insectivore, the tree swallow (Tachycineta bicolor). Low-insect years resulted in decreased tree swallow body mass, with the fitness landscape shifting to favor smaller individuals. Earlier, more temporally variable, and less-pronounced peaks in insect abundance eroded the benefits of phenological synchronization across trophic levels. This phenomenon-which we term trophic decay-led to advantageous phenological mismatch in low-insect years. Our results suggest classic responses to climate change must be evaluated within the context of widespread resource declines.