SUBTLE DIFFERENCES BETWEEN MALE AND FEMALE OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS IN FEEDING ON THE BIVALVE MACOMA-BALTHICA

In this paper an analysis is made of subtle behavioural differences be tween adult male and female Ovstercatchers feeding on Macoma balthica under field conditions and in captivity. Macoma is a tellinid bivalve that in the Dutch Wadden Sea is mainly preyed upon during spring and s ummer when it is buried at a shallow depth. males lift Macoma more, wh ereas females handle them mostly in situ. Both sexes handle a Macoma i n situ faster than one lifted. Time loss of males in handling more lif ted Macoma is compensated by the larger size of lifted Macoma, which y ields more flesh. The time the birds need to find an edible Macoma is similar for both sexes, resulting in equal mean food intake rates for males and females in the field. Lifted Macoma are generally hammered a nd, since males with their short strong bills are more likely to hamme r bivalves than females, this difference in bill morphology might expl ain why males more often lift Macoma than do females, especially as ha mmering produces a blunt bill tip which would reduce efficiency at ope ning Macoma in situ. However, none of the selected bill morphology var iables showed a relationship within the sexes that explained the diffe rences between the sexes.