A specialist in the city: the diet of barn owls along a rural to urban gradient

We investigated the variations in the diets of barn owls (Tyto alba) along a rural to urban gradient in the Lower Fraser Valley of southwestern British Columbia, Canada. Our objectives were to determine the food preferences of barn owls in different habitat types and to assess both seasonal and annual variations in their diets. In particular, given the increased incidences and concentrations of second generation anticoagulant rodenticide (SGAR) residues in barn owls, we were interested in determining whether there was any relationship between the degree of urbanization surrounding their nest/roost sites and the proportion of commensal rodents (Rattus norvegicus, Rattus rattus, Mus musculus) in their diet. In total, 8,941 individual prey remains were identified. The proportion of rats consumed increased significantly with the amount of urbanization within home ranges. However, voles (primarily field voles, Microtus townsendii) were the main prey item for all sites irrespective of surrounding land use within home ranges. Shrews were the second most consumed prey species (10.8 ± 8 %), and were found predominantly in the diet of barn owls nesting in more rural landscapes. The dominance of field voles in the diet was also reflected in the food-niche breadth, which was consistently low for all sites across the rural to urban gradient. Interestingly, barn owls were found nesting in highly urban environments (66–95 % urbanization), where bait stations containing SGARs were regularly used. This, combined with the increased consumption of rats in more urban environments, illustrates why the SGAR exposure rate in barn owls is substantial and rising.