American Woodcock Winter Distribution and Fidelity to Wintering Areas

Abstract

-We examined winter distribution and fidelity to wintering areas for the American Woodcock (Scolopax minor), which exhibits reversed, sexual size dimorphism. Bandrecovery data revealed no difference in winter distributions of different age/sex classes for woodcock from the same breeding ares. Similarly, band recoveries from woodcock banded on wintering grounds revealed no difference in fidelity to wintering sites. Males may winter north of a latitude that is optimal for survival based on physiological considerations, but they gain a reproductive advantage if they are among the first to arrive on the breeding grounds. This may explain our results, which indicate males and females have similar distribution patterns during winter. Received 5 December 1989, accepted 12 May 1990. MANY migratory bird species exhibit sexand age-specific differences in where they winter (see reviews in Ketterson and Nolan 1976, 1983; Nichols and Haramis 1980; Myers 1981). Three hypotheses have been invoked to explain differences. One concerns physiological differences associated with body size, another considers behavioral dominance of age/sex classes, and a third concerns differences in time of arrival on breeding grounds (see Hypotheses and Predictions). Unfortunately, most previous investigations of differential distribution patterns on the wintering grounds have been conducted on species for which the observed distributional differences are consistent with predictions of two or all three of these hypotheses. We investigated winter distribution patterns of American Woodcock (Scolopax minor). They are an appropriate choice for two reasons. First, the hypothesis of behavioral dominance among age/sex classes can likely be rejected a priori for this species, while the two remaining hypotheses yield opposite predictions (see Hypotheses and Predictions). Second, band-recovery data are available. Band-recovery data permit unambiguous inferences about distribution patterns during winter of birds from specific 2 Present address: U.S. Fish and Wildlife Service, Iroquois National Wildlife Refuge, Box 517, Alabama, New York 14003 USA. breeding areas (Nichols and Haramis 1980, Nichols et al. 1983, Perdeck and Clason 1983, Nichols and Hines 1987, Diefenbach et al. 1988a, b). Most studies of winter distribution patterns use samples (e.g. museum specimens) of birds obtained at specific wintering locations without prior knowledge of their origin on the breeding grounds. The geographic variation in age or sex ratios of such samples is ambiguous. Previous investigators have inferred that such variation reflected variation in wintering-ground destination among birds from the same breeding areas. An alternative explanation is that geographic variation in age or sex ratios exists on the breeding grounds and that birds from the same breeding areas migrate together to the same wintering areas (see Nichols and Hines 1987: 35). In this instance, age or sex ratios on the wintering grounds simply reflect ratios on breeding grounds, and proposed explanations of different migration patterns may be unnecessary. In addition to examining winter distribution of the different age/sex classes of American Woodcock, we tested hypotheses about ageand sex-specific variation in fidelity to wintering areas. To date, such questions about fidelity to wintering grounds have been restricted primarily to waterfowl (Nichols et al. 1983, Nichols and Hines 1987, Diefenbach et al. 1988a, b). Our objectives were to test two null hypotheses. First, male and female woodcock from the 745 The Auk 107: 745-749. October 1990 This content downloaded from 157.55.39.220 on Fri, 02 Sep 2016 05:26:05 UTC All use subject to http://about.jstor.org/terms 746 DIEFENBACH ET AL. [Auk, Vol. 107 same breeding areas have the same pattern of distribution in winter. Second, male and female woodcock have similar degrees of fidelity to specific wintering areas. Both of these hypotheses were tested for young, adult, and both ageclasses combined.