Resource abundance can buffer trophic mismatch in a caterpillar-passerine food-chain

Phenological mismatch occurs where variation in the magnitude of the response to environmental cues among species disrupts previously synchronised interspecific interactions, posing a risk to ecosystems as the climate changes. Understanding how ecological and environmental factors modulate the fitness effects of mismatch is essential for forecasting its impacts. Here, we analyse trophic mismatch in a classic woodland caterpillar-passerine system, using fifteen years of data from long-term monitoring sites across England. The timing of peak caterpillar abundance and the abundance of caterpillars varies considerably among sites and years. Applying a Bayesian mixed modelling approach to thousands of nest-level observations, we show that temporal mismatch between chick demand and caterpillar abundance significantly reduces fledging success in Blue and Great Tit but has minimal effect on Pied Flycatcher. We demonstrate that fledging success is greater in years with higher caterpillar biomass, often offsetting the negative impacts of mismatch, but with buffering effects that plateau under extreme caterpillar abundance. Variation in resource availability therefore represents an important but underappreciated buffering mechanism, shaping the fitness consequences of phenological asynchrony in a warming climate.