Geographical Trends in Trophic Characteristics of Mammal-Eating and Bird-Eating Raptors in Europe and North America

Abstract

ABSTRAcr.-If dietary opportunism occurs in predators over large geographic areas, trends in the trophic characteristics of predators should be explained largely by the distribution patterns of their prey. We compiled literature information of diet for mammal-eating and bird-eating raptors (diurnal raptors [Falconiformes] and owls [Strigiformes]) in Europe and North America (299 and 300 samples, respectively) and asked: (1) Do latitudinal and longitudinal patterns occur in the food-niche breadth, number of prey taxa, and prey size within continents? (2) If so, are these patterns consistent with the intracontinental gradients in species diversity and stability (constancy in time) of birds and mammals? In both Europe and North America, latitudinal and longitudinal trends in trophic diversity of mammal-eating diurnal raptors and owls appeared to be more obvious than those of bird-eating and generalist raptors. This suggests that mammal-eating birds of prey are more opportunistic in their diet choice than are bird eaters and generalists. Within Europe, the latitudinal gradient in dietary diversity of raptors was more evident than the longitudinal gradient, whereas within North America the longitudinal gradient was more conspicuous. For both continents, these gradients were more marked in winter diets than in the breeding-season diets of raptors. These results are consistent with the known regional trends in prey assemblages: in Europe, the south-tonorth gradient in diversity and stability of bird and mammal species is more marked than the west-to-east gradient, whereas in North America the number of mammal species markedly increases from east to west, but not so evidently from south to north. Received 6 September 1994, accepted 8 March 1995.