Behaviour of Shelducks Tadorna tadorna in a Winter Flock: Does Regulation Occur?

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Abstract
The hypothesis of Jenkins, Murray et Hall (1975) that the size of the population of shelducks at Aberlady Bay, East Lothian [Scotland, UK], is regulated by competition for feeding space in the late winter flock, the formation of a dominance hierarchy and exclusion of subordinate birds, was tested using individually marked birds, by removal and re-addition experiments, and observations of aggressive interactions and feeding behavior in the winter flock and subsequent territoriality and breeding performance. Experimental removals of birds from the winter flock were not followed by settlement of an equivalent number of birds, even though birds were passing through the area. Re-addition of the birds which had been temporarily removed did not lead to emigration on the level expected if regulation occurred. No consistent dominance was seen in aggressive interactions in the winter flock but an approximate rank order could be allocated on the basis of the proportion of interactions won. Estimates of dominance rank did not correlate with feeding or breeding performance, nor was the rank of the same individual consistent from year to year. No support was found for Jenkins'' hypothesis but an alternative reason was proposed: stability in winter numbers results from a regular annual pattern of arrivals and departures of long-lived shelducks. Evidence from color-marked birds supports this alternative hypothesis.