Both short and long distance migrants use energy-minimizing migration strategies in North American herring gulls

Abstract

Background: Recent studies have proposed that birds migrating short distances migrate at an overall slower pace, minimizing energy expenditure, while birds migrating long distances minimize time spent on migration to cope with seasonal changes in environmental conditions.

Methods: We evaluated variability in the migration strategies of Herring Gulls (Larus argentatus), a generalist species with flexible foraging and flight behaviour. We tracked one population of long distance migrants and three populations of short distance migrants, and compared the directness of their migration routes, their overall migration speed, their travel speed, and their use of stopovers.

Results: Our research revealed that Herring Gulls breeding in the eastern Arctic migrate long distances to spend the winter in the Gulf of Mexico, traveling more than four times farther than gulls from Atlantic Canada during autumn migration. While all populations used indirect routes, the long distance migrants were the least direct. We found that regardless of the distance the population traveled, Herring Gulls migrated at a slower overall migration speed than predicted by Optimal Migration Theory, but the long distance migrants had higher speeds on travel days. While long distance migrants used more stopover days overall, relative to the distance travelled all four populations used a similar number of stopover days.

Conclusions: When taken in context with other studies, we expect that the migration strategies of flexible generalist species like Herring Gulls may be more influenced by habitat and food resources than migration distance.

Keywords: Animal movement; Bird migration; Ecology; Generalist; Migration strategy; Migratory behaviour; Stopover; Telemetry; Tracking.

Fig. 1

Map of migration routes used by Herring Gulls, predicted by state-space models of tracking data. Breeding colonies are represented by a large red square. All individuals were captured at their breeding colony, except three individuals that bred in Newfoundland were captured during the winter in Massachussets, represented by a white star in panel B. Autumn migration tracks are shown in blue for Herring Gulls breeding in a) eastern Arctic (n = 8); b) Newfoundland (n = 12); c) Sable Island (n = 17); and d) the Bay of Fundy (n = 11). Stopover days are represented by yellow circles

Fig. 2

Boxplots illustrating variation in migration characteristics of Herring Gulls in eastern North America. Long distance migrants from the eastern Arctic are shown in grey, and short distance migrants from Newfoundland, Sable Island, and the Bay of Fundy are shown in white. Migration characteristics include a) distance, b) duration, c) start date, d) end date, e) directness, f) migration speed, g) travel speed, h) stopover days, i) travel:stopover ratio, and j) stopover length