Long-term dynamics of breeding waterbirds in relation to habitat structures in a tropical irrigation wetland: evidence from Vettangudi Bird Sanctuary, Tamil Nadu, India

Abstract

Wetlands are critical habitats for breeding waterbirds, yet many are undergoing rapid ecological change due to hydrological alteration, land-use changes, and increasing anthropogenic pressures. We assessed long-term changes in breeding waterbird assemblages in Vettangudi Bird Sanctuary, an irrigation tank in Tamil Nadu, using data from eight breeding seasons (2012–2026). Total breeding waterbird abundance showed high interannual variability with no significant long-term trend. Nest abundance declined over time, from 396 nests in 2012–13 to as low as 163 in 2018–19, with only partial recovery to 190 in 2025–26. Breeding species richness declined, from 11 nesting species in 2016–17 to three species in 2025–26. The breeding community was dominated by Asian Openbill and Black-headed Ibis, while several species, like egrets and herons, showed declining trends or zero nesting in later years. In land-use and land-cover (LULC) analysis, water extent showed a significant negative relationship with breeding diversity during nest initiation (August: ρ = −0.74, p = 0.035), while no significant associations were observed during peak nesting (December). Nesting abundance showed a weak positive association with dense vegetation cover in both August (ρ = 0.43, p = 0.30) and December (ρ = 0.52, p = 0.20). Despite recent increases in water extent and vegetation cover, nesting abundance and species richness declined, suggesting that habitat extent alone does not determine breeding suitability. The observed declines are likely linked to changes in habitat quality, nesting substrate availability, particularly the removal of mature Parkinsonia aculeata trees, coupled with the increasing dominance of Neltuma juliflora. Hence, prioritising targeted conservation, long-term monitoring and management of stable hydrological conditions and nesting trees, and minimising disturbance during the breeding period, are key to sustaining breeding waterbird populations in such vulnerable, human-influenced wetlands.