Young Baltic Gulls take shorter and slower routes to Africa
GPS tracks from 69 birds show juveniles stopping more often and remaining farther north than adults during their first autumn migration.
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Juvenile Baltic Lesser Black-backed Gulls migrate more slowly and are much less likely than adults to cross the Sahara, according to GPS tracks revealing how the Endangered population's journeys change with age.
The Baltic form of Lesser Black-backed Gull is a long-distance migrant using the eastern Afro-Palearctic flyway. Its recent decline has increased the need to understand where birds travel outside the breeding season and whether young gulls face different risks.
Researchers writing in Movement Ecology analysed 57 adults and 12 juveniles tagged at 17 colonies in Sweden. The peer-reviewed early version was published on 3 November.
"Adults tended to migrate further than juveniles, with a much higher probability of crossing the Sahara."
Joan Estrada and study co-authors
Adults completed autumn migration faster even though their average flight speed was lower. The difference came from behaviour between flights: juveniles made longer and more frequent stopovers, extending the overall journey.
Young birds generally remained at more northerly latitudes, while a greater proportion of adults continued to sub-Saharan destinations. The routes taken by both age groups were similarly direct, with little of the exploratory wandering that might be expected if inexperienced birds refined a route largely through trial and error.
The authors suggest social information may help juveniles navigate. Young gulls can travel near other migrants or use established stopover sites, reducing the need for broad exploration. The age difference in destination could also reflect gradual development over successive years, or natural selection favouring shorter migrations among some birds.
Tracking identified important stopover and wintering areas across the flyway. Those locations can now be examined for food supply, disturbance, land-use change, pollution and direct persecution. For a declining population, a bottleneck far from Sweden may affect breeding numbers several months later.
The sample of juveniles was small, and tag survival or loss can bias which full tracks remain available. The study covers birds from Swedish colonies rather than the entire population, and one autumn journey cannot establish whether a juvenile will adopt a more southerly destination later in life.
Mortality away from the colony was not estimated, so the tracks identify places and behaviours that warrant investigation rather than proving which stage of the journey drives the population decline.
Longitudinal tracking is therefore the next test. Following the same birds across several years would distinguish a developmental shift from stable individual differences and reveal when adult-like migration emerges.
The research also cautions against treating a species' migration as a single fixed route. Adults and juveniles can use different sections of the same flyway, stop for different lengths of time and face different conditions. Effective conservation for Baltic Gulls must account for that age structure rather than protecting only the destinations used by experienced adults.
10 Nov 2026
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