Timing mismatches, carryover effects, and the role of neuroendocrine mechanisms in determining birds' responses to environmental change.

The neuroendocrine system plays a critical role in the synchronization of life cycle stages with variation in the environment, and in the coordination of life cycle stages with one another. When humans modify environments, these neuroendocrine mechanisms may impact how different individuals, populations, species, and even communities are affected. Here we conceptualize how endocrine mechanisms may influence the likelihood of: (1) timing mismatches between life cycle stages and environmental conditions, and (2) carryover effects within annual cycles. Timing mismatches can occur when an individual fails to synchronize a particular life cycle stage to the appropriate environmental conditions. Carryover effects occur when activities of one stage (including its timing) affect the performance in one or more subsequent stages. We suggest that there is a trade-off between timing adjustments within and across stages such that neuroendocrine mechanisms that reduce timing mismatches in temporally changing environments (e.g., strong neuroendocrine responsiveness to short-term cues, with resultant increased temporal flexibility to fine-tune the current stage to local conditions) may inherently increase the likelihood of carryover effects (e.g., through delay of a transition between stages), and vice versa. We use two examples-flexibility of the onset of photorefractoriness mediated by responsiveness to short-term cues, and sensitivity of molt to sex steroids-to illustrate these ideas, and suggest that future work should investigate the impacts of variation in these and potentially other seasonal timing mechanisms on carryover effects. The conceptual framework presented here suggests that there may be no single best set of tactics for coping with the effects of climate change; species with neuroendocrine mechanisms facilitating temporal flexibility may avoid some timing mismatches but set themselves up for deleterious carryover effects as they make temporal adjustments to environmental conditions modified by climate change.