Multidimensional plasticity of phenology: Assessing the effects of population density on plastic responses of breeding time to temperature

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.