Introduced predators and nest competitors shape distribution and breeding performance of seabirds: feral pigeons as a new threat.
Petrels are particularly sensitive to predation by introduced species. Many populations have reduced their breeding ranges, currently mainly occupying predator-free sites. Breeding range reduction leads to interspecific competition for nesting sites, which can be detrimental to petrels. 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 Bulwer's petrel Bulweria bulwerii on Tenerife, the largest and most densely human populated of the Canary Islands. We estimated nest density, assessed the role of nest location and physical characteristics of nests on breeding success, and determined causes of breeding failure by introduced predators and competitors. Nest density was higher in predator-free colonies on marine rocks. Cat presence was the best predictor of nest density, but it was not correlated with either presence or abundance of competitors. Breeding success varied between years and colonies but was not related to nest characteristics. Pigeon competition for nests was the most frequent cause of breeding failure (7.3%), followed by rat predation (6.3%). We also compared petrel and pigeon nest cavities and found considerable overlap in the physical size of nest sites. Our study provides insights into 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, as well as disease and pathogen transmission, and vegetation changes within seabird colonies.