GPS-tracking reveals strong individual heterogeneity in migratory movements of Mediterranean Gulls throughout Europe

Non-breeding movements, including migration, can vary greatly among and within avian populations. For instance, many species are partial migrants, including individuals that perform long-distance or short-distance migrations, as well as others that reside around the breeding site year-round. The Mediterranean gull (Ichthyaetus melanocephalus) is an opportunistic feeder and partially migratory species, whose heterogeneity in migratory tactics and year-round habitat use have been poorly investigated. By GPS-tracking more than 50 individuals from populations located in Belgium, France, Italy, and Poland across multiple migration episodes, we investigated the variability in non-breeding movements within and among populations. We characterized non-breeding movement patterns through several parameters and identified a unified framework for characterizing migrants and residents. Gulls showed heterogeneity within and among populations in the direction and distance travelled, while the main wintering sites of migrants across populations were located along the English Channel and the Iberian Peninsula. Our results, the first spanning across the range of the species, revealed that distinct migration tactics coexist within populations and that their frequency can vary among populations. We argue that such behavioural heterogeneity might increase the species’ resilience in the face of rapidly changing environmental conditions.