Temporary flightlessness in pre-laying Common Eiders Somateria mollissima:: are females constrained by excessive wing-loading or by minimal flight muscle ratio?

Large body size, small wings and relatively low flight muscle mass are general attributes of flightlessness in birds, but a general analysis is lacking when considering these factors simultaneously. Common Eiders Somateria mollissima are large sea ducks characterized by short, pointed wings of low surface area. Because females fast throughout incubation, they need to accumulate large body reserves prior to laying. During this pre-laying period, many females cannot take off, and dive when approached under still-air conditions, whereas males take off readily when disturbed. In this paper, we examine how pre-laying female Common Eiders fit the maximum wing-loading ratio of Meunier, the marginal flight muscle ratio (FMR) of Marden and predictions of a general model of take-off performance (also by Marden). Wing morphology was recorded and flight muscles were dissected from specimens collected during the pre-laying period near one breeding colony. In addition, take-off ability, as observed during collection, was compared with the proposed thresholds for flightlessness and outputs from the general model of take-off performance. The results indicated that half of the pre-laying females exceeded the wing-loading ratio of Meunier, although all females had values above 0.160, the flight muscle ratio below which take-off would be impossible. We suggest that wing-loading and flight muscle ratio interact in Eiders, with higher FMR compensating for excessive wing-loading. Nevertheless, the model of take-off performance predicted, with reasonable accuracy, the behavioural observations under still-air conditions. Indeed, females that were predicted to be temporarily flightless could produce a specific lift of 8.8 N/kg on average (less than the 9.8 N/kg required to overcome gravity). In contrast, the average specific lift predicted for males capable of flight was estimated to be 11.4 N/kg. These results agree with our observations that female Common Eiders are at the limit of flight capability in vertebrates.