Limited nest site availability helps seabirds to survive cat predation on islands

Introduced cats Felis catus have a high detrimental impact on native seabirds on islands, especially when alien preys, like rabbits Oryctolagus cuniculus, co-occur. Seabirds are highly vulnerable because of their long reproductive cycles, slow turn-over of generations and the absence of efficient behaviour against terrestrial predators, especially in some burrow-nesting species. Through a deterministic modelling approach, we explored a neglected mechanism that may explain the resistance of some seabird species to cat predation. It was indeed observed that seabirds may compete for nest sites. As a consequence, part of the breeders foregoes breeding when nest sites are a limiting resource. Our model linked the dynamics of cats with that of seabird species. We showed that the annual impact of cats on seabirds was lower when seabirds faced competition for burrows than when the latter were not a limiting resource. This was due to the fact that limited nest site availability prevents an optimal growth of the cat population. Cats in turn cannot manage to exterminate all the prospecting birds during the same breeding season. The limitation of the number of nest sites generates a mechanism leading the bird population to conserve a large pool of sexually mature individuals while only slightly reducing the production of juveniles in the colony. This pool of floaters may play an important role in natural populations by buffering the decrease in colony size during years with harsh environmental conditions on land. In combination with buffer mechanisms, the limitation of the number of nest sites may greatly improve the chances of survival of bird populations facing predation.