Multidimensional phenological plasticity : unravelling the extent of individual variation in wild passerines

This thesis investigates the origins of individual variation in phenological plasticity, using laying dates in birds as a model to understand why individuals differ in how they adjust breeding timing to environmental conditions. Moving beyond single-factor explanations, it examines three complementary dimensions—extrinsic ecological context, intrinsic behavioural strategies, and developmental history—to reveal how these factors jointly shape plastic responses to temperature in the wild. The extrinsic dimension (Chapter 1) tested how ecological conditions at multiple spatial scales influence plasticity in breeding time. Long-term data from a great tit population showed additive effects of spring temperature and density on laying dates: warmer springs and low local density promoted earlier breeding, largely through within-individual plasticity rather than fixed differences among territories. Density did not modify temperature-dependent plasticity, indicating that competitive context shapes baseline timing but not its sensitivity to temperature. Earlier laying increased fledgling success, while lower local density further improved reproductive output. Because density did not interact with temperature to affect fitness, delayed breeding at high density likely reflects constraints rather than adaptive plastic adjustments. The intrinsic dimension (Chapter 2) assessed whether consistent behavioural strategies—specifically exploration behaviour—predict variation in plasticity. Across five European great tit populations, exploration behaviour did not explain differences in reaction-norm slopes: all individuals advanced breeding similarly with warmer temperatures. Personality influenced mean laying dates in only one population, where fast explorers bred slightly earlier. These findings contrast with predictions that behavioural types differ in cue use or flexibility and suggest that temperature cues are so reliable and universally used that within-population behavioural variation has limited influence on plasticity. The developmental dimension (Chapter 3) tested whether early-life conditions leave lasting effects on adult plasticity. Brood-size manipulations combined with long-term monitoring revealed that females raised in reduced broods (“silver-spoon” conditions) showed steeper, nonlinear temperature responses—advancing breeding more in cold springs and levelling off in warm springs—while females from enlarged broods did not differ from controls. Favourable natal environments thus appear to enhance adults’ capacity to respond plastically to environmental variation, potentially by enabling them to bear higher costs of plasticity. Together, these studies demonstrate that phenological plasticity is not a unitary trait but a composite outcome shaped by ecological context, developmental history, and—more weakly—intrinsic behavioural strategies. While temperature-driven plasticity is broadly conserved, its expression varies within populations in structured, context-dependent ways. Understanding these limits and sources of variation is crucial for predicting population persistence under rapid environmental change.