Non-breeding movements of the Black-tailed Gull (Larus crassirostris)

With the continued development of tracking technology and increasing interest in animal movement, our understanding of migration behavior has become more comprehensive. However, there are still many species that have not been well studied, particularly sea birds. Here, we present the first year-round Global Positioning System (GPS) tracking data of the Black-tailed Gull (Larus crassirostris) at the population level. We used solar-powered GPS-Global System for Mobile communication (GSM) loggers to successfully track 30 individuals breeding at Xingrentuo Islet, Liaoning Province, China, for 1–3 years. Except for one individual who roamed in the far north of the Yellow Sea during non-breeding period, all others did a directed southward migration. Migration routes and wintering sites differed among migrating gulls and between years for the birds tracked for two or more years. Additionally, during wintering, the migrating gulls were more likely to travel over a large body of water and shift sites, and some trajectories were quite complex, which was probably closely related to what we observed in the field about their boat-chasing behavior. Compared to wintering movements, the post-breeding movements ranged over a smaller area. Specifically, almost all of them had a long post-breeding period near the breeding islet (≥120 days, <220 ​km from the breeding islet), and 80% of the gulls who were tracked more than one year had at least one faithful post-breeding site. Compared to the post-breeding period, only approximately half of the migrating gulls had a pre-breeding period that was shorter (3–20 days) and closer to the breeding islet (≤80 ​km). Migration distance varied among migrating gulls (range 209–2405 ​km) and the gulls moved least distance during post-breeding period. Furthermore, we found that the southward movement of the migrating gulls occurred when the temperature near the breeding islet dropped; specifically, the gulls directly migrated southward away from the post-breeding site. Our results suggest that the Black-tailed Gull has a long post-breeding period but a short pre-breeding period near the breeding islet and high diversity of their migrating patterns (in especial migration routes and wintering sites).