The ''body size hypothesis'' predicts that if individuals of a population migrate different distances from the breeding to the wintering grounds, the distance should be related to the differential ability to cope with adverse conditions, with larger individuals wintering further north. Data collected over a 40-year period in Essex, UK and the Camargue, southern France, revealed that the average body mass of Teal ringed in Essex during these years was actually not greater than that of Teal ringed in the Camargue. A higher proportion of males were included in the UK ringing catch than in the French catch, but we found no support for the body size hypothesis to explain such differential migration of the sexes.

No abstract

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 1.79 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.

No abstract

Although knowledge of ring recovery rate is of crucial importance to establish demographic parameters, such as survival probability, this information is generally unknown for the dabbling ducks. The almost single existing value from the North American Mallard (Anas platyrhynchos) is thus generally applied to other species or geographic areas, even though this assumption may be strongly misleading. In the study reported here, we have relied on a proxy for actual ring recovery rate, namely, the proportion of rings fitted each year that were eventually reported, to test for differences between duck species (Mallard and Teal Anas crecca) and between countries [England (UK) and France] for Teal. Potential trends over time were also assessed. Ring recovery rate was found to have sharply decreased and at a similar rate over time in both species in France, and in both countries for Teal (e.g. from 22.7% in 1952 to 7.2% in 1992 for the British Teal). Teal rings were, however, reported more frequently than Mallard ones, and Teal rings were more frequently reported in the UK than in France. In recent years, a phone number for reporting the information has been included on the bird mark, with the immediate result of a doubling in the recovery rate in France. Adaptations to current ringing procedures that would improve ring reporting would appear to be necessary in the future if researchers are to keep ringing as a valuable source of information for bird demography studies.