Freshwater wetlands are widely recognized as biodiversity hotspots for many organisms, including birds. Climate change and the projected increased risk of wetland fires may pose a major threat to wetland biodiversity in the future. There is urgent need to assess short- and long-term effect of fires on avian biodiversity and to establish relevant management implications. We analysed the short-term (first 3 months after fire) effect of a large (5 500 ha) spring wildfire on the community of breeding marshland birds in the best-preserved Polish local wetland biodiversity hotspot: Biebrza Valley. We compared the avian community structure and abundance of certain species before and after the fire on the 18 permanent transects located in both burned and unburned habitats. Within first breeding season post fire, fire significantly reduced pooled abundance and species richness of the whole bird community. Three bird species of special conservation concern (including aquatic warbler) temporarily disappeared from burned areas, and the numbers of 11 other species declined. In contrast, only 3 species benefited from the fire, none of which depended on marshes as their primary habitat. Although the reported strong initial fire effect is likely to fade away in subsequent years, its immediate detrimental effects on marshland birds should not be underestimated. We conclude that it is essential to temporarily provide the unburned adjacent refuge areas with additional protection and bird-friendly management and to focus on preventing further degradation of marshes to increase their resilience to fire.