Nest box entrance matters: Nest-site competition in sympatric breeding secondary cavity-nesting birds

Nest-site selection is a critical step in the reproductive process of birds. Limited nest resources lead to intense interspecific competition, which directly affects reproductive outcomes. Currently, there is scant research on how species with a high degree of overlap in breeding time and space achieve sympatric breeding through finer differentiation in nest-site characteristics. In this study, two secondary cavity-nesting birds, the Daurian Redstart ( Phoenicurus auroreus ) and Eurasian Tree Sparrow ( Passer montanus ), were selected as study species. By installing nest boxes with different entrance dimensions (3 cm, 5 cm, 7 cm, and 10 cm × 16 cm) in the study area, we investigated how the two species' use of entrance sizes may contribute to reducing potential nest site competition and explored their defense behaviors toward nest-site competitors. The results revealed that both Daurian Redstarts and Eurasian Tree Sparrows avoided selecting nest boxes with a 10-cm entrance. However, the two species showed different preferences in entrance selection. Eurasian Tree Sparrows preferentially bred in nest boxes with a 3-cm entrance, whereas Daurian Redstarts mainly selected nest boxes with 5-cm or 7-cm entrances. The less competitive Daurian Redstarts exhibited more intense defense behaviors than the more competitive Eurasian Tree Sparrows. This study provides key empirical evidence that nest-site selection is used to cope with interspecific competition between sympatrically distributed secondary cavity-nesting birds. The findings also contribute to a deeper understanding of adaptive behavioral strategies in birds under resource competition and offer a scientific basis for the ecological management of artificial nest boxes.