Chapter 36 – Annual Schedules

Annual schedules of reproduction, migration, and plumage molt evolve to cope with varying environmental predictability, and respond to environmental cues for timing and coordination. Species' schedules may include many or few distinct phenotypic stages; more stages increase tolerance of environmental extremes but decrease temporal flexibility. Proximate factors regulating stages include long-term (photoperiod, endogenous rhythms) and short-term (food, temperature, social factors) predictive information. Response to photoperiod depends on encephalic photoreceptors, an endogenous circadian cycle of light sensitivity, and a transduction system linking these to gonadotropin-releasing hormone secretion, all in the deep brain. Responses to other cues are less well studied, but may either be signalized, depending on sensory processing of the cue, or direct, through metabolism. Effects of these nonphotic cues are usually supplementary to photoperiod (i.e., act in a series, or hierarchy), but in some temporally flexible species different cues may be interchangeable (i.e., may act in parallel). Scheduling of stages can have carryover effects that impact fitness by influencing performance in subsequent stages, even across years. Thus, coordination among stages is as important as coordination with the external environment. Timing mechanisms also affect how populations are affected by climate change; reliance on photoperiod can lead to mismatches between timing of life cycle stages and phenology of the modified environment. Population-level consequences then depend on the rate populations evolve to track the changing environment. Some survival-enhancing processes, such as immune function, must be maintained across all life cycle stages, and trade-offs exist between investment in immunity and energy-demanding life stages such as reproduction, migration, and molt.