Breeding experience and population density affect the ability of a songbird to respond to future climate variation.

Predicting how populations respond to climate change requires an understanding of whether individuals or cohorts within populations vary in their response to climate variation. We used mixed-effects models on a song sparrow (Melospiza melodia) population in British Columbia, Canada, to examine differences among females and cohorts in their average breeding date and breeding date plasticity in response to the El Niño Southern Oscillation. Climatic variables, age and population density were strong predictors of timing of breeding, but we also found considerable variation among individual females and cohorts. Within cohorts, females differed markedly in their breeding date and cohorts also differed in their average breeding date and breeding date plasticity. The plasticity of a cohort appeared to be due primarily to an interaction between the environmental conditions (climate and density) experienced at different ages rather than innate inter-cohort differences. Cohorts that expressed higher plasticity in breeding date experienced warmer El Niño springs in their second or third breeding season, suggesting that prior experience affects how well individuals responded to abnormal climatic conditions. Cohorts born into lower density populations also expressed higher plasticity in breeding date. Interactions between age, experience and environmental conditions have been reported previously for long-lived taxa. Our current results indicate that similar effects operate in a short-lived, temperate songbird.