HULSCHER JB (2006) Sex ration and age composition of wintering Eurasian Wigeon near Haren, Groningen. LIMOSA 79 (2): 41-52.

The sex ratio (% males) of wintering Wigeon near Haren (Groningen) in 2002/2003 was remarkably constant, on average 56.8% from 26 november to 8 april (Fig. 1). This percentage fits in the pattern observed elsewehere in the northern part of the NW-European wintering range. Adult birds left for the winter roosts to the breeding areas a couple of weeks before the juveniles (Fig. 7,8). During cold spells, Wigeons left the roosts as these became covered with ice, and concentrated on nearby waterbodies with partly unfrozen surface (Fig. 9, 10). As soon as ice-cover disappeared, the birds returned to their original roosts. There was no change in the overall sex ratio in the area, suggesting males and females participated similarly in the local movements triggered by the cold spells. These findings are in disagreement with the coldstress hypothesis, stating that the smaller females are in a disadvantage under high energy demands, e.g. during cold weather. The ratio of males to females was higher (not significantly) and the percentage of juvenile males among all males was lower on roosts with high numbers of ducks (range 40-1800) to roosts with smaller numbers (range 44-654). Possibly these differences are due to interactions between the dominant males and subdominant females and juveniles competing for food (dominance hypothesis). It is suggested that in the process of interaction the (usually) restricted feeding conditions near the roosts during daytime may be much more important than feeding opportunities at night, further away from the roosts. The possibility that subdominant females and juveniles are subject to a higher predation risk during feeding is also discussed. Groups of birds with different sex ratios may concentrate at specific places within a roost, resulting from paired birds avoiding unpaired birds. It is concluded (in accordance with other authors), that the present unequal winter distribution of adult male, female and juvenile Wigeons is mainly due to social interactions with dominant males displacing subdominant females and juveniles (dominance hypothesis).

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