Urban development reduces fledging success of Barn Owls in British Columbia, Canada

Abstract

The decline of Barn Owls (Tyto alba) in Europe and North America hasbeen attributed to the loss and fragmentation of grassland foraginghabitat and increased urbanization; both factors can reduce reproductiveoutput and adult survival. We examined how the composition of theagricultural landscape influenced fledging success (defined as thenumber of young fledged per nesting attempt) of a threatened populationof Barn Owls in the Fraser Valley, British Columbia, Canada. Amonglandscape variables, only amount of urban cover was correlated withfledging success of Barn Owls: Fledging success decreased withincreasing urban cover within a 1-km radius of the nest site. Thiseffect was driven by increased brood reduction: Individuals at siteswith more urban cover fledged fewer young but did not lay smallerclutches or fledge young in poorer condition. Given that brood reductionis linked to food availability in Barn Owls and other species, thissuggests that food availability was reduced in more urbanizedlandscapes. Fledging success was not influenced by grassland cover,grassland composition (number and distance to patches of grassland), orthe length of highway within a 1-km radius of nest sites. The proportionof prey biomass consisting of voles varied considerably between nests(range: 0.41-0.92) but was not related to landscape compositionsurrounding a nest site. Because urban cover reduced fledging successbut not diet composition, our data suggest that the amount of urbancover leads to indirect effects on the abundance of small mammals withinthe landscape.