Abstract
Some animals migrate long distances to exploit important seasonal food resources in the northern regions of the northern hemisphere, whilst avoiding winter starvation. Changes in the individual's age and navigational skills are likely to affect migration, which in turn influences the geographic distribution of individuals. Processes such as sexual maturation and navigational abilities are affected by age, and age is thus a key factor in understanding migration patterns and differences in distribution ranges. In the present study, we investigated the effects of age on the geographic distribution of a population of Lesser Black-backed Gulls Larus fuscus throughout its annual cycle, by analyzing a dataset of 19,096 records from 10,000 color-ringed gulls. In contrast to previous assumptions, the results showed that gulls were geographically segregated by age throughout the entire annual cycle, rather than showing a geographic age-related cline only in the wintering areas. This asymmetric distribution results from a reduction in the annual range of sexually mature gulls, and the differential distribution of mature and immature individuals (mature birds remained in more northern areas, compared to immature birds, throughout the annual cycle). Furthermore, although immature gulls travelled longer distances than adults, they initiated their fall migration with short movements, in contrast to adults that migrated using longer movements. The effects identified in this study explain the non-homogenous distribution of populations throughout the annual cycle, with wide implications for the development of effective human health policies and/or wildlife management strategies.
Highlights
The ability to move through a medium is a feature common to many living organisms
Evidence for differential migration patterns within a population has been proposed for several species [7], but these patterns have generally been associated with particular seasons of the year or specific goals
The results point to a clear effect of sexual maturation on the geographic distribution of gulls, with individuals in different sexual maturation classes exhibiting different annual ranges (Fig. 1)
Summary
The ability to move through a medium (such as air, water or mud) is a feature common to many living organisms This movement can range from a few millimeters (e.g., dispersal of bacterial colonies [1]) to thousands of kilometers (e.g., circumpolar migration of arctic terns [2]), and plays an important role in the survival of organisms, at the population level, and at the ecosystem [3] and evolutionary [4] levels. An annual perspective allows a better approach to understanding how interactions among age, external forces, motion, and navigational capacities might be translated into patterns of movement [13] Until recently, such data were almost impossible to collect; the development of appropriate individual tagging techniques has contributed to the appearance of such datasets
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