A framework of hypotheses for studying post-breeding ecology and trade-offs in migratory birds

Abstract Across their annual cycles, migratory birds must manage a complex system of behaviors and physiological transitions to bridge adjoining life history stages. During the transitionary post-breeding period, migrants experience potentially competing demands associated with the end of breeding, molt, and preparation for migration. Here, we describe a framework of nonmutually exclusive hypotheses that reflect potential strategies individuals may use to trade-off the costs and benefits incurred by balancing these 3 events: an Energetic Allocation Hypothesis, where birds overlap the final stages of nesting and postfledging parental care with molt, saving time but experiencing increased energetic and nutritional demands; a Plumage Quality Hypothesis, where birds accelerate molt at the cost of producing poorer quality feathers; a Phenological Delay Hypothesis, where birds delay their migration to avoid these other penalties; and molt-migration, where birds overlap prebasic molt with migration. The costs associated with each of these trade-offs may have significant impacts on individuals and populations when carried over to subsequent stages of the annual cycle. We encourage future studies to use this framework of hypotheses to establish predictions for research on this relatively poorly understood part of the full annual cycle of migratory songbirds.