Northern gannet Morus bassanus foraging ecology: A multidimensional approach

Abstract

This thesis brings together multiple strands of information at different temporal and spatial scales to shed new light on foraging behaviour. Foraging accounts for much of the time and energy budgets of wild animals and underpins many ecological and evolutionary processes. I used the central place foraging trips of a wide-ranging marine predator, the northern gannet Morus bassanus, as a model to study foraging ecology and the influences of fisheries and climate change. My five data chapters span a range of temporal scales, from two-second behavioural classification to interannual variation across 11 years, as well as a range of spatial scales from individual bird-boat interactions to a multi-colony comparison spanning 83% of the latitudinal range of breeding colonies. First, I found sex-specific foraging behaviour across a range of spatial, temporal and isotopic axes and that the extent of segregation varied inter-annually. Second, I examined the energetics of foraging behaviour, revealing the relative costs of different behaviours and how they relate to three-dimensional flight patterns. I found that effort was driven by the time spent performing behaviours rather than differences in the costs of behaviours per unit time. Furthermore, the proportions of each trip spent performing each behaviour were independent of trip length, meaning that foraging trip duration was a good metric of overall effort. Third, I used bird-borne videos cameras to reveal a high incidence of gannets scavenging from fisheries discards and trawl nets in the Celtic Sea. Scavenging is often thought to be an energy-saving strategy, but the energetic cost of scavenging was no different from that of natural foraging. Fourth, I found that gannets did not respond to nearby fishing vessels in Iceland, where discarding is banned and foraging effort suggested plentiful natural prey. Finally, I collated a large multi-colony dataset, which showed that foraging range and trip durations decreased with latitude when controlling for colony size. I related this pattern to poleward range shifting in response to climate change. Overall, I found that foraging behaviour is highly variable in some respects and consistent in others. This is driven by stable factors such as sex and colony location and changing factors such as population size, climate and oceanography, and fisheries practice or policy. This thesis particularly highlights the benefits of combining data from multiple sources, time periods and regions in order to reveal new patterns.