UC Santa Cruz

The non-breeding period is a critical life history period for seabirds, as individuals need to regain body condition lost while breeding. Where animals move is driven by a myriad of factors that can be divided into intrinsic factors, which are linked to physiological and life-history traits, and extrinsic factors which occur as the result interactions with other animals or the environment. To better understand the challenges of wintering in the North Pacific marine ecosystem, I studied the migrations and foraging ecology of three species, surface foraging black-legged kittiwakes (Rissa tridactyla) and red-legged kittiwakes (R. brevirostris), and deep-diving thick-billed murres (Uria lomvia) from three colonies in the southeastern Bering Sea. I tracked birds with geolocation loggers and time-depth recorders over three winters with contrasting environmental conditions (2008-2011). Remotely-sensed environmental data was used to highlight the effects of changing conditions both intra- and inter-annually, while stable isotope values of carbon and nitrogen from feather tissue were used to infer dietary trophic level and niche partitioning. In Chapter 1, I contrasted red-legged and black-legged kittiwake wintering ecology across the spatial, behavioral and dietary niche dimensions and found significant partitioning; red-legged kittiwakes showed greater specialization across all niche dimensions and depended almost exclusively upon on the Bering Sea ecosystem. In Chapter 2, I assessed individual winter foraging strategies of thick-billed murres using a combination of spatial, stable isotope, and behavioral data. Both annual and body size differences largely determined strategies, with some larger bodied birds from St Paul using a deeper daytime diving and more residential strategy unexploited by smaller birds. In Chapter 3, I contrasted intrinsic drivers (e.g. sex, breeding colony and experience) with the influence of annual habitat conditions on black-legged kittiwake winter distributions. I found large annual differences in distributions that spanned the sub-arctic North Pacific, yet in consecutive winters individuals returned to the same regions more often than expected. Overall, environmental conditions played a significant role in influencing the movements and behavior of individuals, however life history characteristics in consecutive winters such as sex, dietary preference or body size sometimes constrain individual wintering ecology.