Effects of the Cardiff Bay tidal barrage on the abundance, ecology and behaviour of shelducks Tadorna tadorna

1. Closure of the Taff/Ely Estuary by the Cardiff Bay barrage in 1999 resulted in the replacement of intertidal mudflats by a permanent freshwater lake. This led to an 89% reduction in the population of shelducks Tadorna tadorna. 2. The birds switched from foraging mainly for Nereis diversicolor and Hydrobia ulvae by scything with the bill as they walked across the mudflats at low tide, to feeding on benthic chironomid midge larvae while swimming in shallow water around the margins of the lake. 3. The population decline occurred as a consequence of a decrease in the area available for foraging — from about 1 km2 of mudflats to about 0.1 km2 of water shallow enough for shelducks to reach the bottom when dabbling, head dipping and upending. 4. Contrary to expectation, the amount of time shelducks spent feeding was similar pre‐ and post‐barrage, and their body and plumage condition improved. 5. A tidal rhythm in activity persisted, with a reduced amount of feeding at high tide, probably because of the slight rise in water levels that sometimes occurred at this time. Copyright © 2008 John Wiley & Sons, Ltd.