Distribution and Breeding Success of Pallid Swifts, Apus pallidus, on Mediterranean Islands: Nest Predation by the Roof Rat, Rattus rattus, and Nest Site Quality

Abstract

Nest site quality and egg predation are two important components of breeding success in birds. Here we study the role of pallid swift nest site quality and of predation by rats in explaining swift abundance and distribution on Mediterranean islands. First, we monitored the breeding success in natural nests on a rat free island in relation to physical features of nest sites and to meteorological conditions in order to understand the impact of weather on breeding success. Second, we inferred the impact of nest predation by rats empirically from the comparison of the distribution of rats and swifts on the islands. Third, we studied the impact of rats on swift nests experimentally by using artificial nests baited with quail eggs. Artificial nests were placed in sites mimicking extreme types of nest sites naturally used by swifts in relation to their accessibility to small mammalian predators. The experiment with artificial nests were conducted simultaneously on rat infested and rat free islands. Results show that the effect of meteorological events such as heavy summer rains on breeding success depends on the physical features of the nest site (depth, protection, number of exits). We speculate that while breeding success of some nests is affected by heavy summer rains these nests are at an advantage in case of unusually high temperatures. Experimental and observational results on rat predation show that rats dramatically reduce or extirpate local swift populations on rat infested islands. Only a small number of nest sites originally available to swifts will provide sufficient protection from predation by rats and are still used by swifts on rat infested islands. Rats and weather interact in a complex fashion on these islands in shaping the observed pattern of nest site selection and breeding success of swifts. The results also suggest that rats probably heavily influence the local diversity of bird communities and the regional distribution of many bird species.