The migration is energy-demanding and is expected to greatly affect the distribution of individuals over the species range and condition the choice of migratory routes. We investigated the wintering distributions and migratory flyways use of geographically contiguous populations of Lesser Black-backed Gulls (Larus fuscus) and difference in population winter age structure between migratory flyways. Recoveries of metal ringed pulli from Denmark, Sweden, and Finland were used. The results showed that contiguous populations can have distinct wintering distribution patterns and migratory flyways. More importantly, we found that depending on the place of origin, the population winter distribution may or may not show a latitudinal cline in the age structure. The population migrating via the eastern Atlantic flyway (western flyway) showed a winter age-related latitudinal cline, with adults staying at more northern latitudes than immatures. In contrast, no such pattern was found in the population migrating along the Mediterranean/Black sea flyway (eastern flyway). Interestingly, immatures within the eastern population showed a more dispersed pattern of migratory bearings. Overall, our results enhance the importance of the migration flyway in shaping the age structure of populations in the winter quarters and how it may influence the effect of other factors like sexual maturation. 1. Introduction During migration, geographically distinct populations of the same species can face different challenges when moving between breeding and wintering areas. Ecological barriers, such as the Sahara Desert, or mountain ranges, such as the Alps or the Pyrenees, contribute to the shaping of migratory routes and the patterns of distribution of the species that have to cross them [1, 2]. Thus, in species that are spread over wide geographical ranges, populations from the extremes of the distribution can have different migratory problems to solve and can thus develop distinct migratory behaviours in response to the specific ecological constraints (e.g., different migratory routes or migration timings [3–5]). A longitudinal effect on the wintering distribution of populations has been well documented [6–9]; birds that breed furthest west in the breeding range tend to winter furthest west in the nonbreeding range, while those that breed furthest east also winter furthest east. This pattern represents a general tendency revealed by ringing recovery data for most groups of birds [1, 2]. Similarly, latitudinal effects can also be found within and among populations of the same species
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