White-shouldered starlings prefer nest boxes with smaller entrance to avoid nest usurpation

This study examined nest site selection preferences of the white-shouldered starling (Sturnia sinensis), a relatively small species for its medium body size, using artificial nest boxes to mitigate the risk of nest usurpation.This study was conducted in Tanniu Town (19° 69′ N, 110° 77′ E), Wenchang City, Tropical Hainan Island, south China. The area has a tropical marine climate with an average annual temperature of 23.9°C, 1,953.8 h of annual sunshine, and annual precipitation ranging from 1,529.8 to 1,948.6 mm. This region represents a relatively closed ecosystem primarily focused on livestock and freshwater aquaculture (Liu et al., 2023).Four secondary cavity-nesting bird species are predominantly present at the site, namely the white-shouldered starling, with a body length of 170–200 mm and a body mass of 37–51 g; the crested myna (Acridotheres cristatellus), with a body length of 210–277 mm and a body mass of 78–150 g; the common myna (A. tristis), with a body length of 240–260 mm and a body mass of 100–118 g; and the oriental magpie robin (Copsychus saularis), with a body length of 178–227 mm and a body mass of 32–50 g (Zhao, 2001), while the Eurasian hoopoe (Upupa epops) prefers to nest in the holes of walls of buildings in local farms (Soler et al., 2024).Before the installation of artificial nest boxes, these secondary cavity-nesting birds built their nests at the tops of coconut trees or in crevices in building walls. However, due to large areas of land being utilized for aquaculture and livestock farming, the availability of natural tree cavities in the area has significantly decreased. This scarcity of natural nest sites provided ideal experimental conditions for attracting birds using artificial nest boxes (see Liu et al., 2023). The four species share similar diets (Zhao, 2001), and white-shouldered starlings experience intense competition for cavity resources with the other three species, particularly crested mynas and common mynas. Sharing similar nesting materials, white-shouldered starlings, being the most slender-bodied species, are likely to face the highest risk of nest usurpation.Results revealed that white-shouldered starlings preferred nest boxes with smaller (4 cm) entrance over larger (6 cm) ones. This preference likely reduces competition with larger bird species and minimizes the risk of nest usurpation. Additionally, successful breeding in a nest box increased the likelihood of its reuse by the same species. The physical condition of parent birds also influenced nest site selection; individuals with shorter tarsus lengths and bills were more likely to choose smaller entrance, which were inaccessible to larger species, rather than double-entrance boxes. This study highlights the ecological importance of nest site selection as a strategy for small birds to manage interspecific competition, and provides valuable insights into the mechanisms underlying coexistence among sympatric conspecific and heterospecific birds.