Data from: Body stores persist as fitness correlate in a long-distance migrant released from food constraints

Long-distance migratory birds rely on acquisition of body reserves to fuel their migration and reproduction. Breeding success depends on the amount of body reserve acquired prior to migration, which is thought to increase with access to food at the fuelling site. Here we studied how food abundance during fuelling affected time budgets and reproductive success. In a regime of plenty, we expected that (1) limitations on food harvesting would become lifted, allowing birds to frequently idle, and (2) that birds would reach sufficient fuel loads, such that departure weight would no longer affect reproductive success. Our study system comprised brent geese (Branta b. bernicla) staging on high-quality agricultural pastures. Fuelling conditions were assessed by a combination of high-resolution GPS-tracking, acceleration-based behavioural classification, thermoregulation modelling, and measurements of food digestibility and excretion rates. Mark-resighting analysis was used to test for correlations between departure weight and offspring recruitment. Our results confirm that birds loafed extensively, actively postponed fuelling in early spring, and took frequent digestion pauses, suggesting that traditional time constraints on harvest and fuelling rates are absent on modern-day fertilized grasslands. Nonetheless, departure weight remained correlated with recruitment success. The persistence of this correlation after a prolonged stopover with access to abundant high-quality food, suggests that between-individual differences in departure condition are not so much enforced by food quality and availability during stopover, but reflect individual quality and longer-lived life-history traits, such as health status and digestive capacity, which may be developed before the fuelling period.