. The Canadian field-naturalist. Natural history. 2001 TWEDT, LlNZ AND BLEIER: GEOGRAPHIC ORIGIN OF YELLOW-HEADED BLACKBIRDS 553. September Figure 2. Proportion of male and female Yellow-headed Blackbirds classified as migrants in central North Dakota during 1987 and 1988. Plotted proportions are running averages based on 5-collection-day samples. Mean sample sizes were 54 (range 15-134) for females and 78 (range 29-166) for males. North Dakota (Twedt et al. 1994), thereby increas- ing the difficulty of distinguishing differences between these populations using either principal components anal
. The Canadian field-naturalist. Natural history. 2001 TWEDT, LlNZ AND BLEIER: GEOGRAPHIC ORIGIN OF YELLOW-HEADED BLACKBIRDS 553. September Figure 2. Proportion of male and female Yellow-headed Blackbirds classified as migrants in central North Dakota during 1987 and 1988. Plotted proportions are running averages based on 5-collection-day samples. Mean sample sizes were 54 (range 15-134) for females and 78 (range 29-166) for males. North Dakota (Twedt et al. 1994), thereby increas- ing the difficulty of distinguishing differences between these populations using either principal components analysis or discriminant function analy- sis. In summary, although distinct morphometric dif- ferences exist between Yellow-headed Blackbirds breeding in central North Dakota and those breeding at more northwestern locations in Canada, we were unable to exploit these differences to elucidate either the duration or magnitude of migration through central North Dakota. Unless refinement of these morphometric techniques can be achieved, it appears that elucidating migratory movements of this species will continue to rely on physically marking individual birds or the development of improved genetic markers. Acknowledgments We thank B. Osborne for his dedicated assistance in the field and laboratory. Additional laboratory assistance was provided by J. Lindlauf, and B. Mautz. We are deeply indebted to the numerous landowners who granted access to their property. Additionally we thank personnel from state, provin- cial, and federal wildlife agencies for their coopera- tion. Funding was provided by the USDA National Wildlife Research Center and North Dakota State University. Literature Cited Aldrich, J. W., and F. C. James. 1991. Ecogeographic variation in the American Robin (Turdus migratorioiis). Auk 108: 230-249. Atwood, J. A. 1989. Inferred destinations of spring mi- grant common yellowthroats based on plumage and mor- phology. Pages 377-383 in Ecology and conservation of neotropical mi
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