There's no place like home: tropical overwintering sites may have a fundamental role in shaping migratory strategies

Abstract

Migration theory has largely overlooked the role of overwintering grounds and yet many migratory animals spend most of the annual cycle there. High winter site familiarity could confer considerable advantages on animals preparing for subsequent migration phases, enabling them to maximize migration speed and minimise the use of unknown stopover sites. Despite this, quantitative studies of the magnitude of fuel stores attained at such sites are lacking. To evaluate the energetic contribution of an overwintering site to spring migration, we used manual and automated telemetry to estimate departure date, departure fuel load, flight range and breeding destination of blackpoll warblers (Setophaga striata) wintering in Colombia. Relative to overwinter mass, warblers increased body mass (a surrogate for fuel stores) by an average of 59% prior to departure from the overwintering site. Flight range (mean 2710 km, range 2110–3380 km, N = 16) and detections at automated telemetry stations in North America suggest movements to Cuba/southeastern US, before refuelling and then continuing towards breeding grounds in eastern Canada (total duration ∼ 30 days). Total migration length was ∼ 5350 km and energy acquired at the overwintering site was sufficient to cover 39–63% (mean = 50%), demonstrating the major contribution tropical wintering grounds can make to migration. It follows that varying the extent of fuelling prior to departing overwintering sites may provide animals with a mechanism for balancing their exposure to risk during migration, while simultaneously allowing for flexibility in migratory schedules in a changing world.