Diet and trophic niche of the Slender-billed Barn Owl (Tyto alba gracilirostris) on Lanzarote

Abstract The feeding ecology of the endangered Slender-billed Barn Owl ( Tyto alba gracilirostris ), a subspecies endemic to the Canary Islands, is studied based on pellets from northern Lanzarote. A total of 577 g of material were examined, yielding at least 438 prey individuals comprising at least 14 species from 10 families of Mammalia, Squamata, Insecta, and Aves. The diet was dominated by the House Mouse ( Mus musculus , 55.25%) and the Canary Wall Gecko ( Tarentola angustimentalis , 23.97%). Considering the biomass, a wide spectrum of prey individuals weighing between 0.1 and 800 g was found. The calculated Shannon Index (1.33) and Evenness (0.51) indicate moderate prey diversity, whilst the standardised Levin’s niche breadth index ( B norm = 0.128) reflects narrow dietary specialisation. Despite the wide spectrum of biomasses, the median prey mass was 19 g and the arithmetic mean was 26.76 g, suggesting a functional preference for prey individuals in this lower weight class. However, the full exploitation of locally abundant taxa in the resource-poor island ecosystem stands against the assumption of strict specialisation.