Three-dimensional foraging behaviour of Northern gannets

Movements of animals form a vital link between individual behaviour and spatial processes affecting populations, communities and ecosystems. Predators often have to search for prey that is unpredictable or patchily distributed and increasingly have to do so in habitats that are changing as a result of climate change or anthropogenic activity. Our understanding of animal movement has been revolutionised by the development of miniaturised loggers which allow tracking of individual animals over a range of spatial and temporal scales. However, while flight heights of birds are a critical component of avian movement ecology, they have been little-studied in comparison to horizontal movements. In this thesis I examine the three-dimensional foraging behaviour of northern gannets (Morus bassanus) at the world’s largest breeding colony, at Bass Rock, Scotland. I first combine GPS and pressure data to estimate flight heights, and develop a novel refinement that uses sea surface pressure data to correct estimated heights during long periods of sustained flight. I then investigate sexual differences and effects of weather on three-dimensional foraging behaviour, before examining potential population-level consequences of mortality from collisions with offshore wind turbines. I found that sexual differences in foraging behaviour extend to the heights at which birds fly, and that wind speed in particular has a marked effect on movements and behaviour at sea. I also found that predicted levels of mortality from offshore wind farms would be likely to retard population growth but unlikely to drive the population into long-term decline. My thesis describes some of the complexity and flexibility of gannet foraging behaviour and highlights the importance of understanding movements in three dimensions.