Mismatched breeding phenology of a North American migratory songbird is adaptive

Phenological advances are among the most widely reported consequences of anthropogenic climate warming. Phenotypic plasticity causes phenological adjustments to increasing temperatures that result from climate change as well as to naturally high inter-annual variability. A trophic mismatch can occur if plasticity differs among interacting species such that an organism becomes temporally decoupled from its food. This decoupling has been linked to population declines and is often used as a measure of adverse climate change impacts. However, life history theory predicts that unequal phenological plasticity across trophic levels may be adaptive. We used 25 years of measurements of an open-cup nesting, migratory songbird (the black-throated blue warbler, Setophaga caerulescens) to study the timing of their arrival and breeding, and its consequences for reproductive success. Spring leaf phenology (partial r² = 0.64), food abundance during the egg formation period (partial r² = 0.05), and breeder age (partial r²  =0.06) all correlated with the timing of first clutch initiation. Breeding was not timed with maximum food availability, which did not show a predictable seasonal peak. Spring arrival of birds was weakly related to leaf phenology (0.16 ± 0.08 d/d), but plasticity in the timing of breeding was stronger (0.56 ± 0.08 d/d). Although not isometric with leaf phenology, the observed plasticity in breeding time matched that which maximized fitness, as predicted by life history theory. The ability to appropriately adjust breeding phenology to inter-annual climatic variation caused reproductive output to vary by up to 1.8 offspring per pair (52% of average annual production), roughly equal to the variation in reproductive output caused by food and density dependence, but less than that caused by nest survival. Individual females differed in lay date under average conditions (sd = 1.9-4.3 d), but understanding whether heritable responses to environmental cues or non-heritable physiological constraints underlie these differences is essential for predicting evolutionary responses to climate change.