Nesting ecology of the Asian Openbill (Anastomus oscitans) in an urban landscape: a case study from Gaya, India

The Asian Openbill (Anastomus oscitans) is a colonial, tree-nesting waterbird species that nests in both wild woodlands and urban landscapes across India. However, its nesting site selection within urban environments is not yet fully understood, despite being a critical aspect of its life history. In this study, we investigated the nesting ecology of the Asian Openbill in the urban landscapes of Gaya city (Bihar), India, from June to October over two consecutive years (2023–2024). We observed a single colony within a 0.24 km2 area, which contained 3556 active nests on 224 trees of 20 different species; the breeding population reached a maximum of 7112 individuals. The majority of nests (82.96%) were constructed on 14 different native tree species. Nesting tree preference was strongly skewed towards Ficus religiosa (25.53%), followed by Mangifera indica (9.90%) and Ficus racemosa (9.67%). Nesting trees presented a significantly greater height, girth at breast height (GBH), and canopy area than non-nesting trees did, with positive correlations between nest number and both the GBH and canopy area. The nest density reached 751 nests/0.1 km2 and peaked in August. Although environmental variables such as temperature, humidity, rainfall, and day length were temporally correlated with nesting peaks, statistical analysis using a Generalized Linear Model revealed no significant direct effects of these variables on nest number. These findings underscore the ecological significance of a small urban area in Gaya city for the nesting of Asian Openbill and highlight the importance of specific tree characteristics within this limited yet ecologically valuable area, which has important implications for urban biodiversity conservation and sustainable urban planning.