Evidence of Barred Owls (Strix varia) Depredating Nests of Barn Swallows (Hirundo rustica)

Abstract

From 2010–2012, I studied nesting success and causes of nest failure at a colony of Barn Swallows (Hirundo rustica) where anecdotal evidence suggested the colony was declining in size. In 2010, many nests were destroyed, and indirect evidence suggested breeding adults were being depredated by a raptor. Using trail cameras, I captured video evidence of Barred Owls (Strix varia) depredating two nests, one containing chicks and the other eggs. During the study period, 72% (32/44; 2010), 51% (16/31; 2011), and 85% (17/20; 2012) of nests were depredated, and the number of nesting attempts declined by more than 50% over 3 years. The video evidence and plucked adult feathers suggests that Barred Owls were responsible for at least some of the predation. Barred Owls did not breed historically on the southern British Columbia coast but first appeared in 1966 and have become common breeders, while Barn Swallows have decreased in the same region by 5.64% per year since 1970. There may be many causes of local and regional population declines of Barn Swallows, but given their conspicuous nests, predation in less well protected nesting sites may be a localized cause of colony decline.