Climate change affects the duration of the reproductive season in birds.

1. The duration of the reproductive season may depend on the duration of the growing season, with recent amelioration in spring temperatures allowing earlier start of reproduction. Earlier start of reproduction may allow a longer breeding season because of more broods a longer interval between broods for multi-brooded species. 2. We analysed extensive long-term data sets on timing of breeding in 20 species of birds from Denmark, based on records of over 100 000 individual offspring, showing considerable heterogeneity among species in temporal change in duration of the breeding season. 3. Multi-brooded species increased the duration of their breeding season by 0.43 days year(-1) while single-brooded species decreased the duration of their breeding season by 0.44 days year(-1). This implies that recent climate change has allowed more broods or better temporal spacing of broods in multi-brooded species, while the time window for reproduction has become narrower in single-brooded species. 4. The single-most important predictor of change in duration of the breeding season was change in the date breeding started; there was no change in the date of end of breeding. Species advancing their breeding date the most also expanded the duration of the breeding season. In contrast, long-distance migration and generation time did not predict change in duration of the breeding season.