Changes in phenology in response to global warming are often unequal across functional groups, leading to phenological mismatches between predators and prey. In our study population of great tits (Parus major) in the Netherlands, advancements in laying dates in response to warmer springs have been insufficient to keep pace with changing caterpillar phenology. Many females now breed too late relative to the seasonal peak in caterpillar abundance. This mismatch has intensified selection for earlier breeding, but the impacts on population demography are poorly understood. We statistically examined relationships among phenological mismatch, natural selection, and population dynamics using 38 years of individual-level data.
Directional selection for earlier laying was stronger in years where birds bred on average later than the food peak (strong population mismatch). However, population mismatch had a weak effect on the mean number of recruits per female, and no effect on mean adult survival, after controlling for the effects of breeding density and other environmental variables on these key vital rates. There was no overall relationship between the annual strength of directional selection and mean population fitness, and numbers remained relatively stable over this period of pronounced regional warming. These findings illustrate how climate change can affect individual relative fitness without significantly impacting population dynamics, at least via phenology and within the observed range of environmental conditions. Laying date is heritable in our study population but not under density-dependent selection. Thus, while evolution of laying dates might influence population viability (an ‘evo to eco’ linkage) if the mismatch becomes more extreme under future climate change, we do not expect changes in population dynamics to feed back on evolutionary dynamics.