Foraging behaviour and population dynamics of northern gannets over a period of environmental change

There is an urgent need to understand better how recent climatic changes and shifting marine environments are affecting the population dynamics and foraging behaviour of marine central-place foragers. I use both single and multi-colony, and single and multi-species approaches to investigate how different aspects of colonial breeding in seabirds impact on parental foraging behaviour under these changing environmental conditions. I combine historical and recent colony counts and trip duration data to consider population-level interactions and relationships for northern gannets Morus bassanus. I also consider the role of intra-specific competition in limiting neighbouring colony growth for three additional North Atlantic seabirds, the Atlantic puffin Fratercula arctica, European shag Phalacracorax aristotelis and black-legged kittiwake Rissa tridactyla, with differing population trends over the past three decades. I then proceed to focus on finer-scale effects on individual foraging behaviour and parental care of northern gannets at one colony over a period of 14 years, using direct observations and a variety of bird-borne logging devices.

My findings support models of foraging based on competition for prey around seabird colonies. I also found that annual variation in foraging trip durations was particularly marked at large colonies, making them especially vulnerable to adverse effects of low prey availability at sea. Furthermore, as foraging ranges altered with colony sizes, the scale of apparent interactions between conspecifics at neighbouring colonies also altered, providing novel support for the limiting effect of neighbouring conspecific density on population growth. Gannets also showed annual flexibility in diet, habitat use and finer-scale search strategies, although other aspects of their foraging, such as the mean scale of Area Restricted Search behaviour, appeared less flexible. Further study is now required to understand if gannets have the capacity to alter the scale of their search behaviour under more extreme conditions. Gannets were also flexible within breeding seasons in the foraging and parental effort they expended, suggesting that the chick’s requirements also contribute to adults’ foraging behaviour. Flexibility in foraging and parental behaviour may buffer the potential adverse impacts of variable environments on provisioning and productivity. These findings have implications for the population dynamics and potential resilience of a wide range of seabird species and other central-place foragers.