Browsing by Author "Sowter, David"
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- Differential annual movement patterns in a migratory species: Effects of experience and sexual maturationPublication . Jorge, Paulo; Sowter, David; Marques, Paulo A. M.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.
- Gulls can change their migratory behavior during lifetimePublication . Marques, Paulo A. M.; Sowter, David; Jorge, PauloMigration is a widespread phenomenon among birds and is likely to be subject to strong selective pressures. Birds’ annual routines and behaviors might be expected to change during their diff erent life history stages, resulting in diff erent, agerelated migration patterns. However, although migration has been the subject of many publications, age-related diff erences in migration have received little attention. Th e present study examined age-related changes in individual migration habits in lesser black-backed gulls, Larus fuscus. We analyzed data from 10-year (1998–2007) color-ringing project in NW England, comprising more than 10 000 ringed individuals. Our results showed a latitudinal cline in age structure across the wintering range, with adults and gulls in their fi rst breeding year wintering closer to the breeding grounds. Supporting this result we observed that individuals, as they get older, changed the migration behavior and winter closer to the breeding areas. Interestingly, we found no diff erences in survival rates across the wintering grounds. Th us diff erences in survival rates can not account for the latitudinal cline in age structure, and the observed fi ndings seem to be best explained by the arrival time hypothesis, based on a mechanism whereby individuals are able to change their migratory behavior as result of the onset of sexual maturity and associated mating pressures.