Factors affecting breeding distribution and seabird richness within the Azores archipelago

Seabird populations in the Azores archipelago are currently much smaller and more restricted in distribution than in the past. Important factors in this decline include predation by alien mammals, human exploitation, and habitat loss. We investigated the extent to which the presence of human and introduced predators, and some geographical features of the islands affect distribution and richness of seabirds breeding on this archipelago. Richness of seabird species (five Procellariiformes, one gull and two tern species) was higher on the main islands, which possess cliffs. As a result, shearwaters and gulls were more likely to be found on the larger islands that also tended to have rats and cats present. However, Madeiran Storm-petrel Oceanodroma castro and Bulwer’s Petrel Bulweria bulwerii only breed in numbers on a very few rat-free islets. Continued management is needed to avoid human disturbance and alien invasion onto islets with small petrels. We recommend study of the effects of mammals on Little Shearwaters Puffinus assimilis baroli and Manx Shearwaters P. puffinus in the Azores, as the overlap between the distributions of these two species and rats is surprising.