Changes in breeding phenology and population size of birds

1. Although the phenology of numerous organisms has advanced significantly in response to recent climate change, the life history and population consequences of earlier reproduction remain poorly understood. 2. We analyzed extensive data on temporal change in laying date and clutch size of birds from Europe and North America to test whether these changes were related to recent trends in population size. 3. Across studies, laying date advanced significantly while clutch size did not change. However, within populations, change in laying date and change in clutch size were positively correlated, implying that species that advanced their laying date the most were also those that increased their clutch size the most. 4. Greater advances in laying date were associated with species that had multiple broods per season, lived in non-agricultural habitats and were herbivorous or predatory. The duration of the breeding season increased for multi-brooded species and decreased for single-brooded spe...