Body mass loss amongst moulting Pochard Aythya ferina and Tufted Duck A. fuligula at Abberton Reservoir, South East England

We assessed the contribution of endogenous fat stores to meeting energetic needs during the flightless moult period in Pochard and Tufted Duck by regressing mass on the progression of wing moult, as measured by primary length. Pochard lost between 22.1% (males) and 24.2% (females) of body mass and female Tufted Duck 12.2% during wing moult at Abberton Reservoir, Essex. Based on a 27-day flightless period, Pochard lost on average 8.3–8.4 g per day and Tufted Duck 3.2–3.4 g per day, presumably due to fat expenditure. Assuming the daily energy expenditure (DEE) to be 1.7× the basal metabolic rate (BMR), the contribution from burning endogenous fat equated to 37–40% of Pochard and 19–20% of Tufted Duck DEE during remiges moult. One frequently re-trapped female Pochard expended fat stores that almost fulfilled her entire energetic requirements during moult if she rested and did not feed for most of the moult. These results confirm that Pochard and Tufted Duck accumulate fat stores that help to meet energetic needs during the flightless wing moult period. This endogenous source of energy may free them to exploit habitats that are safe from predation but which may not fully supply the energetic needs of moulting ducks from exogenous sources.