Water level manipulations in human-made impoundments drive species- and guild-specific responses in wetland bird communities

For over half a century, wildlife managers have constructed impoundments to offset wetland habitat loss across North America. As impoundments age, productivity often declines, and water-level drawdowns are commonly used to rejuvenate habitat and primary productivity and increase wildlife use, particularly targeting wetland bird communities. We evaluated the effectiveness of drawdowns in coastal impoundments of Atlantic Canada, where responses to management are poorly understood. From April–June 2021–2023, we used a replicated before–after–control–impact (BACI) design to assess bird occupancy and abundance at untreated, drawdown, and post-treatment wetlands using generalized additive and mixed-effects models. We also tested the response of aquatic macroinvertebrates to drawdown. Bird responses varied among foraging guilds and species. Shallow-water foragers, including aquatic ground predators and some dabbling ducks, increased during drawdown, whereas aquatic diving species declined. Following refilling, overall bird abundance increased significantly at treatment wetlands relative to controls. Aquatic macroinvertebrate abundance did not increase after drawdown, suggesting that bird responses were more strongly linked to changes other than standing prey biomass. These findings indicate that drawdowns can enhance wetland bird use but generate trade-offs among species with different functional traits and habitat requirements. Managing impoundments on a rotating drawdown schedule to maintain habitat heterogeneity at a landscape scale may help maximize biodiversity and support a broader range of wetland birds in Atlantic Canada.