Résumé
Phenotypic plasticity is the key adaptive mechanism behind annual adjustment of breeding time in response to temperature. In nature, organisms are not only subjected to variation in temperature but encounter multiple fluctuating environmental factors that affect phenotypic expression, including conspecific density, which affects individual performances through resource competition. We examined the interactive effects of temperature and conspecific density at two spatial scales (territory and patch level) on breeding time and success utilizing data obtained from long‐term monitoring of a wild great tit Parus major population in a fragmented woodland. As expected, we detected earlier breeding in response to warmer spring temperatures. We report earlier laying at low territory level density (i.e. a larger available area per breeding pair), but no evidence of density effects at patch level (breeding pairs per hectare). Birds experiencing low territory level density throughout their life bred on average earlier, and this response was also seen at the within‐individual level (earlier laying in years when individuals experienced a lower density than average). We found no context‐dependence of plastic responses to warmer springs as we detected no interactive effects between density and temperature. In terms of breeding success, earlier laying decreased the risk of brood failure and increased the number of fledglings. The number of fledglings was higher at low territory level density, while higher patch level density increased the probability of brood failure. Altogether, these results indicate that density‐related responses were likely mediated by food competition rather than by increased numbers of low‐quality birds or increased occupation of poorer territories at higher densities. This study highlights the importance of examining parameters at different spatial scales, along with the study of individual responses to multivariate cues for a comprehensive understanding of the variations in phenological plasticity.