Inter-colony movements, at-sea behaviour and foraging in an immature seabird: results from GPS-PPT tracking, radio-tracking and stable isotope analysis
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Seabird populations contain large numbers of immatures––in some instances comprising >50% of the fully grown adults in the population. These birds are significant components of marine food webs and may contribute to compensatory recruitment and dispersal, but remain severely understudied. Here, we use GPS-PTTs, radio-tracking and analysis of stable carbon (δ13C) and nitrogen (δ15N) isotopes to investigate the movements and foraging ecology of immature seabirds. Our study focussed on immature northern gannets Morus bassanus aged 2–4 attending non-breeding aggregations alongside a large breeding colony. GPS-PTT tracking of five birds revealed that immatures have the ability to disperse widely during the breeding season, with some individuals potentially prospecting at other colonies. Overall, however, immatures were faithful to the colony of capture. During returns to the focal colony, immatures acted as central place foragers, conducted looping and commuting flights, and analysis of the variance in first-passage time revealed evidence of area-restricted search (ARS) behaviour. In addition, stable carbon (δ13C) and nitrogen (δ15N) isotope analyses indicate that immatures were isotopically segregated from breeders. Our findings provide insights into the foraging, prospecting and dispersal behaviour of immature seabirds, which may have important implications for understanding seabird ecology and conservation.
We would like to thank Greg & Lisa Morgan, Tim Brooke at VentureJet, Anthony Bicknell, Valentina Lauria, Carrie Gunn, James Waggitt, Claudia Stauss and Simon Rundle for help in field, laboratory and for comment on the manuscript. The Royal Society for the Protection of Birds granted permission to work on Grassholm. Device attachment was conducted with permission of the Countryside Council for Wales. SCV was funded by a NERC New Investigators Grant (NE/G001014/1), WJG funded by a studentship from the Peninsula Research Institute for Marine Renewable Energy and SP funded by EU INTERREG Project CHARM III.
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