Abstract
Studying the at-sea movements and behaviour of juvenile seabirds is logistically challenging, but new technologies now allow tracking birds on their first migration, giving a more complete picture of population-level spatial ecology. We investigated the post-fledging migration of juvenile northern gannets Morus bassanus from the world’s largest colony, at Bass Rock, Scotland. We first examined the movements and survival of 38 juveniles over their initial days at sea with GPS precision for up to 53 d post-fledging. We then compared their migration journeys with those of 35 adults tracked with geolocators. Almost one-third of juveniles died within 2 mo of leaving the colony, and this mortality was often associated with apparent uncertainties in their direction of migration, including marked, abrupt and often repeated changes in bearing within the North Sea. Both juveniles and adults then migrated as far as the Canary Current Large Marine Ecosystem (CCLME) off the Atlantic coast of West Africa, initially taking both clockwise and counter-clockwise routes around the UK. Juveniles covered a distance each day similar to that of adults, but they reached the CCLME much more quickly, mainly because they travelled more directly, staying close to the coast throughout most of their migration, whereas adults additionally spent long periods over relatively restricted areas of ocean further offshore. The CCLME is a hotspot of unregulated fishing activity, and our findings highlight the importance of this region across different age-classes of birds, echoing previous calls that the regional strengthening of marine conservation should be a high priority.
Highlights
Young animals typically have much higher mortality than adults, and understanding this difference is fundamental to the study of population structure, size and dynamics (Charlesworth 1980, Stearns 1992, Genovart et al 2018)
Understanding the movements of naïve individuals has been identified as a priority area for marine research due to a need to better understand how juveniles disperse from natal areas, make use of different marine habitats and overlap with potential threats (Hazen et al 2012, Riotte-Lambert & Weimerskirch 2013)
Argos locations from GPS-PTTs on juveniles were filtered by speed and location class (LC) to remove erroneous locations using the R package ‘argosfilter’ (Freitas 2012, Langston et al 2013; speeds > 25 ms−1 and LC Z were removed)
Summary
Young animals typically have much higher mortality than adults, and understanding this difference is fundamental to the study of population structure, size and dynamics (Charlesworth 1980, Stearns 1992, Genovart et al 2018). Juveniles are independent of their parents after fledging (Nelson 2002) and need to learn critical foraging, flight and navigation skills without parental guidance, possibly contributing to high mortality during their first year (Wanless et al 2006). They may remain on the water for several days post-fledging while they lose mass and gain sufficient strength to take off from the water surface (Wanless & Okill 1994), yet little is known about their subsequent movements or behaviour. We tracked birds with GPS precision for up to 53 d post-fledging, allowing us first to examine the movements and survival of juveniles over their initial days at sea, and to compare their migration journeys with those of adults tracked with geolocators from the same colony, examining migration routes and speeds of travel
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