Abstract
Individual wintering strategies and patterns of winter site fidelity in successive years are highly variable among seabird species. Yet, an understanding of consistency in timing of movements and the degree of site fidelity is essential for assessing how seabird populations might be influenced by, and respond to, changing conditions on wintering grounds. To explore annual variation in migratory movements and wintering areas, we applied bird-borne geolocators on Thick-billed Murres (Uria lomvia, n = 19) and Common Murres (U. aalge, n = 20) from 5 colonies in the Northwest Atlantic for 2–4 consecutive years. Thick-billed Murres ranged widely and among-individual wintering strategies were highly variable, whereas most Common Murres wintered relatively near their colonies, with among-individual variation represented more by the relative use of inshore vs. offshore habitat. Within individuals, some aspects of the wintering strategy were more repeatable than others: colony arrival and departure dates were more consistent by individual Common than Thick-billed Murres, while the sizes of home ranges (95% utilization distributions) and distances travelled to wintering area were more repeatable for both species. In consecutive years, individual home ranges overlapped from 0–64% (Thick-billed Murres) and 0–95% (Common Murres); and the winter centroids were just 239 km and 169 km apart (respectively). Over the 3–4 year timescale of our study, individuals employed either fixed or flexible wintering strategies; although most birds showed high winter site fidelity, some shifted core ranges after 2 or 3 years. The capacity among seabird species for a combination of fidelity and flexibility, in which individuals may choose from a range of alternative strategies, deserves further, longer term attention.
Highlights
Many seabirds migrate seasonally to wintering areas where foraging and environmental conditions, affected by natural and anthropogenic processes, can influence their population dynamics [1,2]
The repeatability (r) of standardized departure dates was low for Thick-billed Murres (r = 0) and high only for Common Murres from Gannet and Funk Islands (r = 0.56, 0.82, respectively; Table 2)
Departure dates between consecutive years differed on average by 6.765.3 days in Thick-billed Murres and by 5.764.6 days in Common Murres
Summary
Many seabirds migrate seasonally to wintering areas where foraging and environmental conditions, affected by natural and anthropogenic processes, can influence their population dynamics [1,2]. These influences can be direct, through mortality, or indirect, through carry-over effects of winter body condition on subsequent reproduction [3,4]. Among the few studies of seabirds that tracked the same birds repeatedly, some species showed high wintering-site fidelity [1,5,12,14], whereas in others, individuals shifted wintering locations between years [16,17,18]. Individual behavioural flexibility will influence the capacity of populations to cope with rapid climatic and habitat changes [17,21]
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