Does prey abundance affect prey size selection by the Eagle Owl (Bubo bubo)?

Abstract

Optimal foraging theory predicts that in situations of high prey abundance, predators will select the largest ones if the cost of capture is similar. This relationship is not clear in species of large owls. We aimed to evaluate the selection by the Eagle Owl (Bubo bubo) of different body sizes of a profitable prey, the European Rabbit (Oryctolagus cuniculus). Evaluations were made in a semi-natural rabbit population with high rabbit availability (Los Melonares), and in two natural populations with different abundances of rabbit (low to high). In Los Melonares, we compared the size distribution of rabbits in the owl diet with the body size distribution of the rabbit population. In the natural populations, we compared the size of the rabbits in the owl diet with the abundance of rabbits. Results from Los Melonares showed that the Eagle Owl positively selected smaller rabbits (< 300 g) regardless of rabbits size distribution within the population throughout the year but such selection was only significant when the abundance of rabbits reached the highest levels. On the contrary, owls avoided rabbits belonging to the largest size class (> 900 g) throughout the year, being not significant in winter with low abundance of rabbits. In natural populations, smaller rabbits were included in the owl diet more frequently when rabbit abundance increased. Thus, Eagle Owl seemed to select the smallest rabbits, probably due to their higher vulnerability, ease of capture, their predictable location in areas surrounding warrens, and the lesser effort required to handle, consume and deliver to nestlings.