Drivers of seabird distribution and foraging movements
Keywords: seabirds
behaviour
Balearic shearwaters
distribution
random forest
generalized additive models
penguins
foraging

Recent improvements in biotelemetry and analytical methods have increased our understanding of the at-sea movements and behaviour of elusive pelagic seabirds, many of which are becoming the focus of conservation efforts. In this thesis, I consider several issues related to the environmental correlates of the foraging movements and at-sea distributions of a critically endangered shearwater in UK waters and of two penguin species in Antarctic and sub-Antarctic waters; I also consider the use of the ‘dual foraging strategy’ (i.e., both short- and long-distance foraging) during chick-rearing of different seabird taxa.
This thesis explores the associations of oceanographic variables and prey abundance with the at-sea distribution of both non-breeding seabirds (Balearic shearwater Puffinus mauretanicus, Chapter 2) and of breeding seabirds (macaroni penguins Eudyptes chrysolophus, Chapter 4). Chapter 2 combines 5 years of vessel-based surveys of post-breeding Balearic Shearwaters in the western English Channel and southern Celtic Sea with simultaneously collected bio-acoustics data of fish abundances and remote-sensed environmental data. I map Balearic shearwater presence across the area using both Generalized Additive Models (GAMs) and Random Forest (RF) models, providing the first estimate of the distribution of this critically endangered seabird in UK waters, information that is essential for the designation of a Special Protection Area (SPA). Oceanic variables associated with prey distributions were better predictors of seabird movements than direct measures of prey distribution, possibly because snapshots of prey availability cannot capture the dynamic nature of prey distributions.
Using data from GPS and time-depth recorders, this thesis also investigates the presence and dynamic nature of prey depletion around breeding colonies in two penguin species (chinstrap penguins Pygoscelis antarcticus in Chapter 3, and macaroni penguins in Chapter 4) and estimates foraging patch quality based on time allocation during foraging dives. Foraging patch quality increased with increasing distance from the colony for both species, suggesting that birds depleted prey near their colony, providing evidence for “Ashmole’s halo”. However, neither species utilized the dual foraging strategy. Interestingly, habitat quality near the chinstrap penguin colonies improved over the breeding season, suggesting that chinstrap penguins, like some other seabirds, time their reproduction so that the period of maximum resource availability in the environment aligns with their period of maximum demand for resources.
Finally, the drivers of dual foraging in different seabird families are identified in a systematic review (Chapter 5) that compiles published literature on the use of dual foraging by seabirds during chick-rearing with concurrent estimates of habitat quality around the colonies. We found the likelihood that seabirds use the dual foraging strategy increases with increasing magnitude of the difference in the quality of the habitat accessible during short- versus long-distance foraging trips. However, there were significant differences in the use of dual foraging across taxa: Albatrosses and Penguins were significantly less likely to dual forage than Procellariidae, while Sulids were significantly more likely to dual forage than Procellariidae. This suggests that other, taxon-specific factors – such as travel costs and the resilience of chicks against starvation – also play a role in determining the use of the dual foraging strategy by different seabird taxa.
Overall, these findings advance our understanding of the environmental drivers of seabirds’ foraging distributions and provide valuable information for their conservation.