Inference studies from body mass during the non-breeding season are rare in seabirds due to methodological difficulties to sample individuals. Here, we used 12 years of data collected from 2000 to 2019 on about 6800 birds mist-netted at La Somone, Senegal along the East Atlantic Flyway to examine the role of intrinsic and environmental factors in explaining body mass variation in both Common Sterna hirundo and Roseate S. dougallii Terns. Most captured terns were subadults and adults aged three years and older, although ca. 22% of trapped Common Terns were immatures aged one–two years. Mean body mass values of Common and Roseate Terns were substantially lower than published values reported from European breeding sites. In both species, females were, on average, lighter than males, and in the Common Tern, immatures were also lighter than older individuals. Based on the subset of recoveries of birds initially marked at birth in Europe, we found significant differences in body mass according to natal origin in Common Terns but not in Roseate Terns. The variations reported for the former species were mainly attributed to size, with lighter and smaller individuals originating from the British Isles, intermediate individuals from western/central Europe, and heavier and larger birds from Fennoscandian countries. Capture date, used as a proxy measure of timing of migration, showed a strong covariation with body mass in both species. In fact, as the spring season progressed, mist-netted birds became progressively heavier, irrespective of age and natal origin. This suggests that earlier and lighter migrants may have opted for a more protracted migration duration involving more frequent and/or longer refueling periods. Conversely, late migrants needed greater body reserves to achieve the return journey to the colonies within a shorter period to arrive on time for the breeding season. Global climate phenomena such as the North Atlantic and Southern Oscillation indices did not explain inter-annual variations in body mass during the course of this study. Overall, our results suggest that, during the non-breeding season, body mass in these two long-distance migratory species is mainly shaped by intrinsic factors and inter-individual variation in migration strategy.
