GEOGRAPHICAL TRENDS IN TROPHIC CHARACTERISTICS OF MAMMAL-EATING AND BIRD-EATING RAPTORS IN EUROPE AND NORTH-AMERICA

If dietary opportunism occurs in predators over large geographic areas , trends in the trophic characteristics of predators should be explain ed largely by the distribution patterns of their prey. We compiled lit erature information of diet for mammal-eating and bird-eating raptors (diurnal raptors [Falconiformes] and owls [Strigiformes]) in Europe an d 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 speci es diversity and stability (constancy in time) of birds and mammals? I n both Europe and North America, latitudinal and longitudinal trends i n 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 Euro pe, the latitudinal gradient in dietary diversity of raptors was more evident than the longitudinal gradient, whereas within North America t he longitudinal gradient was more conspicuous. For both continents, th ese gradients were more marked in winter diets than in the breeding-se ason diets of raptors. These results are consistent with the known reg ional trends in prey assemblages: in Europe, the south-to-north gradie nt in diversity and stability of bird and mammal species is more marke d than the west-to-east gradient, whereas in North America the number of mammal species markedly increases from east to west, but not so evi dently from south to north.