Length of stopover, fuel storage and a sex-bias in the occurrence of red knots Calidris c. canutus and C-c. islandica in the Wadden Sea during southward migration

During southward migration the Wadden Sea is the meeting place of Red Knots Calidris canutus of two subspecies that breed in either western Siberia (C . c. canutus) or north Greenland and north-east Canada (C. c. islandica), b ut the details of their co-occurrence have not been described. In 1995-98 n umbers of Red Knots in our study area in the western Dutch Wadden Sea usual ly built up in late July towards maxima of 10 000-20 000 individuals in Aug ust and early September. In each of these four years we attached tiny (1.3- 1.8 g) radiotransmitters to a total of 95 molecularly sexed adults to deter mine the length of stay of different categories of birds. The 65 females (6 8%) predominated the samples, and among the females the majority (48 birds) was captured without traces of wing moult. In females, but not in males, b irds caught in wing moult stayed significantly longer than non-moulting bir ds. Non-moulting females weighed up to 200 g and disappeared within three w eeks after being marked. The timing of their disappearance corresponded wit h observed departures of flocks towards the southwest, and published depart ure times of canutus. The relationship between length of stay and mass at c apture of these early departing non-moulting females suggests a daily mass gain of about 2.84 g d(-1). These birds had a mean bill length that was 1 m m (yet significantly) longer than those of the other female categories; a r elatively long bill is a well known attribute of canutus. The much smaller sample of males with similar mass, moult and staging time characteristics d id not show longer bill lengths and we are thus unable to unambiguously con firm the presence of canutus males in late July and early August; this bias remains to be functionally explained. Sex ratios were even in birds assign able to islandica.