Feral pigeons as a nest competitor for a small seabird

Abstract Due to their life-history traits, the petrels are particularly sensitive to predation by introduced species. Therefore, many populations have constricted their original breeding distribution range, currently only occupying predator-free sites. In this scenario, interspecific competition for nesting sites can be detrimental for the petrel’ conservation. Here, we evaluate how the presence of introduced mammals (cats Felis catus and rats Rattus spp.) and potential competitors for nest sites (Cory’s shearwaters Calonectris borealis and feral rock pigeons Columba livia) shape the distribution, breeding density, and breeding performance of the Bulwer’s petrel Bulweria bulwerii on Tenerife, the largest and most densely populated of the Canary Islands. We estimated nest density, assessed the role of nest location and nest characteristics on breeding success, and determined causes of breeding failure by exotic predators and competitors. Nest density was higher in predator-free colonies on marine rocks. Cat presence was the best predictor for nest density, while presence or abundance of competitors had no correlation to nest density. Breeding success varied between years and colonies, but was not related to nest characteristics. Of the unequivocally determined causes of breeding failure, pigeon competition for nests was the most frequent (7.3%), followed by rat predation (6.3%). We also compared petrel and pigeon nest cavities and found a considerable overlap in nesting niche. Our study provides insight on an overlooked impact of the invasive rock pigeon: nest competition with small seabirds. We encourage more research on the effects of pigeons on nest density, disease and pathogen transmission, and vegetation changes within petrel colonies.