The diet of Atlantic Yellow-legged Gulls ( Larus michahellis atlantis ) at an oceanic seabird colony: estimating predatory impact upon breeding petrels

The diet and breeding ecology of Yellow-legged Gulls ( Larus michahellis atlantis ) were studied on Selvagem Grande, North Atlantic in the nesting season of 2007. We collected and analyzed 715 pellets from adults. The most frequent prey were White-faced Storm-petrels ( Pelagodroma marina ; present on 40.8% of all pellets) and the endemic land snails ( Theba macandrewiana ; present on 36.5% of all pellets). Other birds, namely Cory’s Shearwaters ( Calonectris diomedea ), Macaronesian Shearwaters ( Puffinus assimilis ), Bulwer’s Petrels ( Bulweria bulwerii ), and Band-rumped Storm-petrels ( Oceanodroma castro ) were relatively less frequent, but overall, seabirds were present in ca. 50% of all pellets, representing an estimated 60.4% of all mass consumed by gulls. We estimate that the contribution of seabirds to the overall caloric balance accounted for 82.5% of all energy consumed. The number of gull pairs breeding on Selvagem Grande was 12 on 2005 and 2007. Breeding success was low (0.92 and 0.25 juveniles per breeding pair, respectively). Using a simple bioenergetics model, we estimate the breeding gull population to have the potential to consume approximately 4,847 adult/sub-adult seabirds in 3.5 months in order to meet its energetic requirements. The importance of the estimated predation levels is discussed and some management actions are suggested.