Waterbirds persisted while terrestrial birds collapsed: contrasting habitat-guild responses to reservoir desilting in a wintering bird assemblage, western India

Abstract Desilting and dredging are widely used to restore the storage capacity of shallow tropical reservoirs, and their ecological effects are usually assessed in terms of waterbirds. We monitored the bird assemblage of Pariyej reservoir, Gujarat, western India, across four consecutive wintering seasons (November–March, 2022–2023 to 2025–2026; ten visits per season) spanning a desilting programme during which the reservoir was drained from February 2024 and refilled after the following monsoon. Species were assigned to five habitat guilds, and seasonal comparisons used the November–February core wintering window. We recorded 154 species in 18 orders and 54 families. Per-visit richness fell from 117.1 ± 4.6 to 47.8 ± 2.3 species and recovered to 115.5 ± 3.1 (Kruskal–Wallis H = 26.09, p < 0.001; ε 2 = 0.825), with the first and last season’s statistically indistinguishable. Sample coverage exceeded 99.9% throughout, so these changes were not detection artefacts. The decline was strongly guild-dependent and ran opposite to expectation: scrub- and tree-dependent birds lost 74% of per-visit richness and 88% of abundance, whereas open-water birds ended the study at 129% of baseline richness and 160% of baseline abundance. The wetland-dependent share of individuals rose from 63.9% to 89.0% during the works before returning to 65.9% (H = 26.41, p < 0.001). Season explained 87% of compositional variance (PERMANOVA pseudo-F = 60.33, p = 0.0001). The assemblage did not lose its waterbirds; it lost the terrestrial community dependent on fringing woody vegetation, which conventional waterbird-focused impact assessment would not have detected.