Why does the Daurian Redstart prefer exhaust fan rain covers for nesting?

Abstract

The Daurian Redstart Phoenicurus auroreus, formerly considered a winter resident in Japan, is expanding its breeding area. Although this species was originally a cavity-nesting bird, it has been selecting nesting sites in exhaust fan rain covers, with wider openings than birdhouses and mailboxes. To understand the reasons, we compared two types of exhaust fan rain covers, one that birds used and one that they did not, focusing on the view from the nest. As a result, the species was found to use exhaust fan rain covers with significantly smaller vertical viewing angles or solid angles, regardless of the size of the opening. In addition, exhaust fan rain covers had the characteristically narrow fields of view from the nest, making it difficult for potential predators to enter the field of view. Therefore, we suggest that the Daurian Redstart used exhaust fan rain covers as nesting sites to avoid detection by predators. Investigating the field of view from the nest using a solid angle is useful for evaluating the nesting site selection patterns.