| Abstract: | Predation may reduce prey numbers in such extent that prey may be depleted, which has negative effects on predator populations. Prey depletion would be more likely when the number of predators increase and/or concentrate their activity in a certain area, as is the case of colonial birds. As a matter of fact, food depletion due to intraspecific competition is considered a major cost of coloniality, and several studies have shown indirect evidence of this. However, no direct measures of food depletion have been provided along with its consequences for the fitness of the colony inhabitants. We carried out a field study with the lesser kestrel Falco naumanni, a raptor that breeds in colonies ranging from two to dozens of pairs. During the nestling period we sampled the main prey of the kestrels around different sized colonies at increasing distances. At the same time, we recorded hunting distances and prey delivery rates to the nest. In addition, we monitored the reproductive success in colonies of different sizes. Lesser kestrels feed their nestlings mainly with grasshoppers and these prey became depleted through the season in the surroundings of the large colonies. Prey depletion made kestrels fly longer distances to forage and prey delivery rates to the nest decreased. Lower feeding rates were not compensated by bringing larger prey, hence, the net amount of energy provided to the chicks decreased with the date in large colonies. By contrast, none of this occurred around small colonies, where both prey abundance and hunting distance remained constant throughout the season. As a consequence, the seasonal decline in the reproductive success (number of fledglings and fledgling body condition) was greater the larger the colony. Thus, these results evidence that food depletion and its fitness costs are related to colony size, as they are suffered by the kestrels breeding in large colonies but not by those settled at small ones. Finally, the consequences of prey depletion on the demographic dynamics and the regulation of colony size are discussed. |
