Diet specialization in an insular population of coastal Peregrine Falcons

Individual diet specialization is known to occur in populations of generalist predators, where specific individuals develop specialist feeding strategies. Diet specialization has been reported in many raptor species, and it may be an important driver of intraspecific population structure. Here, we quantify the diet of five breeding pairs of Peregrine Falcons Falco peregrinus from an offshore island determined from prey remains collected over four breeding seasons. Three prey species accounted for 69.8 % of total prey frequency, with Manx Shearwater Puffinus puffinus the primary prey accounting for 47.3 % by frequency and 40.8 % by biomass. Herring Gull Larus argentatus was the second most important prey species by frequency (13.8 %) and biomass (29.8 %) followed by Domestic Pigeon Columba livia (frequency = 8.7 %, biomass = 7.0 %). Predation frequency on specific prey groups varied substantially between breeding pairs and months. Two pairs specialized on Manx Shearwater, one pair specialized on Herring Gull and Manx Shearwater, with the remaining two pairs having a relatively generalist diet of Manx Shearwaters, Domestic Pigeon and small passerines. Predation on Manx Shearwaters increased throughout the breeding season with a peak in total diet frequency of 63.8 % in July, with a concurrent decrease in Herring Gull predation frequency. Higher percentage of Manx Shearwater in the diet was able to explain 87 % of the variation in a narrower dietary breadth for the Peregrine pairs. Our results suggest individual diet specialization may be important for understanding population density in insular raptor populations.