The at-sea behaviour and ecology of the critically endangered Balearic shearwater

Abstract

Seabirds are long-lived, diverse and behaviourally complex marine top predators that are capable of traversing large areas of the global oceans. Consequently, this group are at risk from the wide and persistent range of anthropogenic activities working in this environment. Understanding the consistency with which individuals and populations use the marine environment over space and time, and the mechanisms underlying at-sea behaviour is therefore vital for interpreting population dynamics and developing appropriate and long-standing conservation strategies.

This thesis utilises a combination of state-of-the-art tracking technologies and biogeochemical analyses to provide a better understanding of the at-sea movements, ecology and behaviour of a critically endangered seabird: the Balearic shearwater Puffinus mauretanicus. This species is the most threatened seabird in Europe and is undergoing continued population declines, believed to be largely associated with at-sea mortality from fisheries bycatch and predation on land. Despite intensive study during the breeding season, knowledge of the at-sea ecology of Balearic shearwaters during key phases of the annual cycle remains poor.

Year-round tracking from colonies on the Balearic Islands yielded new insights into migration strategies of individuals and populations. Most individuals remained faithful to non-breeding areas over the course of the five-year study, although some plasticity in migration behaviour was also detected, indicating capacity for change. Patterns of differential migration were persistent in the main study population, and were linked to sex-based (and potential life stage-based) differences in migratory behaviour, which are most likely associated with varying ties to the breeding grounds. Links between reproductive performance and non-breeding behaviour were also detected, demonstrating an importance of carry-over effects in this species, with potential implications for population dynamics. Furthermore, behavioural differentiation was found between island populations. Migration strategies, use of foraging habitat and phenology differed between a potential Balearic/Yelkouan shearwater hybrid population on Menorca and a neighbouring colony of Balearic shearwaters on Mallorca, providing insights into the relatedness of Puffinus species in the Mediterranean, and emphasising the need to identify units for management that are both ecologically and evolutionarily relevant.

Combined geolocation, isotope and feather moult data further identified use of a diversity of foraging tactics in northeast Atlantic waters, and spatial differences in non-breeding dietary behaviour. These findings implicate a role of both forage fish and fisheries in shaping patterns of at-sea distribution during the non-breeding season, and may prove useful for future assessment of seabird responses to anthropogenic and environmental change. During the breeding season, persistent use of highly productive coastal habitats was identified, indicating exploitation of predictable resources. Such movements emphasise the vulnerability of the Balearic shearwater to anthropogenic activity, but also highlight the potential of area-based management approaches for species protection, when combined with management of human activities throughout the species’ distribution range.

Together, the findings of this research provide urgently needed information on the at-sea behaviour and ecology of the Balearic shearwater, which should contribute to improved management efforts aimed at increasing population viability. In addition, this thesis contributes to a wider understanding of individual behaviours and inter-seasonal interactions in seabirds, and identifies the need to establish the movement behaviour of a wider range of life stages and populations across distinct seasons.

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