Abstract
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.
Highlights
Petrels occur in vast ocean areas, but their breeding colonies are usually restricted to islands that are markedly limited in area in comparison with their large pelagic feeding grounds (Brooke 2018)
Sympatric species fiercely compete for favourable nesting sites, with the smaller species usually defeated (Cory’s shearwater Calonectris borealis vs. Bulwer’s petrel Bulweria bulwerii, Macaronesian shearwater Puffinus baroli, and band-rumped storm-petrel Hydrobates castro; Ramos et al 1997; Wedge-tailed shearwater Ardenna pacifica vs. Tahiti petrel Pseudobulweria rostrata; Villard et al 2006; Streaked shearwater Calonectris leucomelas vs. H. castro and Swinhoe’s storm-petrel Hydrobates monorhis; Sato et al 2010; Bonin petrel Pterodroma hypoleuca and A. pacifica vs. Tristam’s storm-petrel Hydrobates tristami; McClelland et al 2008), differences in microhabitat or nest site characteristics can partially alleviate interspecific competition (Sullivan and Wilson 2001; Bourgeois and Vidal 2007; Troy et al 2016)
Nest density was higher in colonies without predators like cats or rats, but it was not related to nest competitors such as feral rock pigeons or Cory’s shearwaters (Fig. 2)
Summary
Petrels (order Procellariiformes) occur in vast ocean areas, but their breeding colonies are usually restricted to islands that are markedly limited in area in comparison with their large pelagic feeding grounds (Brooke 2018). Petrels are in decline mainly due to predation by introduced species on their breeding grounds, being one of the most endangered avian groups (Rodríguez et al 2019; Dias et al 2019). Especially petrels, nest underground, and share nesting grounds with other species. Sympatric species fiercely compete for favourable nesting sites, with the smaller species usually defeated (Cory’s shearwater Calonectris borealis vs. Bulwer’s petrel Bulweria bulwerii, Macaronesian shearwater Puffinus baroli, and band-rumped storm-petrel Hydrobates castro; Ramos et al 1997; Wedge-tailed shearwater Ardenna pacifica vs. Tahiti petrel Pseudobulweria rostrata; Villard et al 2006; Streaked shearwater Calonectris leucomelas vs. H. castro and Swinhoe’s storm-petrel Hydrobates monorhis; Sato et al 2010; Bonin petrel Pterodroma hypoleuca and A. pacifica vs. Tristam’s storm-petrel Hydrobates tristami; McClelland et al 2008), differences in microhabitat or nest site characteristics can partially alleviate interspecific competition (Sullivan and Wilson 2001; Bourgeois and Vidal 2007; Troy et al 2016)
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