Seasonal Sex and Age Ratios in Populations of the White-Crowned Sparrows of the Race Gambelii

King, James; Farner, Donald; Mewaldt, L

Populations of White-crowned Sparrows of the race Zonotrichia leucophrys gambelii were sampled during migration and on the wintering grounds in southeastern Washington and in central California, and on the breeding grounds in central Alaska. The data are analyzed in terms of seasonal changes in the proportions of adult and of immature birds and of males and females within each age class. On the breeding grounds near Fairbanks, Alaska, the sex ratio in adults did not differ significantly from unity during the postnuptial period in 1961; the mean values for immature birds may indicate a slight preponderance of males. The postnuptial population near Fairbanks probably contained between 60 and 70 per cent immature birds. During autumn migration through southeastern Washington, near the northern margin of the winter range, the combined data for 12 years reveal a steady increase in the proportion of males in the samples during the migratory season, varying from about 25 per cent males at the beginning of migration to about 55 per cent males at the end of migration for both adult and immature birds. Although the pattern is interpreted as indicative of a later migration of males in both age classes, use of a hypothetical model shows that data obtained at only a single site cannot verify this interpretation. The differential migration of the sexes in autumn could be geographic, temporal, or both. On the average, samples of migrant flocks in southeastern Washington contain between 70 and 80 per cent immature birds during most of the migratory period, increasing to about 85 per cent during the terminal days of migration. However, there is considerable annual variation in age ratio. The most obvious interpretation of this is that it reflects a delayed migration of at least part of the immature population. Nevertheless, alternative explanations involving geographic rather than temporal differentiation are equally valid. On the wintering ground in the Snake River Canyon in southeastern Washington, the population was stable at about 80 per cent males from November through March. There were no statistically significant differences in the sex ratio in the two classes tending to spend the winter farther south. Our samples from California plus additional data from the literature and from museum specimens confirm this and suggest also that females may tend to winter predominantly east of the Sierra Nevada and to northwestern Mexico. On the average, the winter population in the Snake River Canyon contained about 70 per cent immature birds. Winter populations in central California approximated 60 per cent immatures. Spring migration in southeastern Washington began with flocks consisting of approximating 95 per cent males. At the end of migration the sex ratio had decreased to approximately 50 per cent males. Presumably this change resulted from an earlier or more rapid migration of males. The first flocks to arrive in central Alaska consist exclusively of males. The earliest females may not arrive until two weeks later.

Seasonal Sex and Age Ratios in Populations of the White-Crowned Sparrows of the Race Gambelii

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