Breeding ecology of the Asian openbill in eastern Nepal: Larger trees support higher fledgling success.

Abstract

Colonial nesting waterbirds in agricultural landscapes have historically received limited research attention, especially in South Asia. For example, the Asian openbill (Anastomus oscitans) is a colonial species that extensively utilizes agricultural landscapes, yet there is a notable lack of substantial studies despite increasing urbanization across these landscapes. We investigated the factors affecting the breeding ecology of Asian openbills in eastern Nepal. We used a grid-based approach to locate stork colonies and monitored them throughout the breeding season from May to November for three consecutive years (2020-2022). Altogether, we observed a total of 67 active colonies, comprising 4020 active nests, which successfully fledged a total of 7566 chicks. Additionally, most of the colonies were located within areas of human settlements (40%), followed by community forests (33%) and agricultural land (27%). The Asian openbills primarily preferred large trees, such as Bombax ceiba (72%), for nesting. The mean height of nesting trees was approximately 4 m taller, the diameter at breast height was twice as large, and the canopy cover area was three times greater than that of non-nesting trees. The canopy cover area of trees positively influenced the colony size, while colony size positively influenced the fledgling's success. Our study underscores the significance of large trees in providing sufficient space for accommodating a substantial number of openbill nests and fledglings. These findings have significant implications for conservation efforts to protect large trees along with wetlands and agricultural lands, as crucial measures to ensure the sustainable breeding of this nationally vulnerable species.