Seasonal, Annual, and Geographic Variation in Sex Ratio of Wintering Populations of Dark-eyed Juncos (junco Hyemalis)

--Previously published data pooled over entire winters showed clinal variation in the winter sex ratio of Junco h. hyemalis. Females were less numerous than males in the north and progressively more numerous southward. In this study, regression coefficients calculated from the published data, with latitude and certain climatic measures as independent variables, accurately predicted the winter sex ratio of locations not previously sampled. Annual variation in sex ratio was nonsignificant. The pattern of distribution was established by the end of fall migration, before differential mortality could have accounted for it. Temperature rather than latitude, snowfall, or probable distance from the nearest breeding grounds correlated best with percentage of females in winter populations. It is suggested that climate has acted primarily indirectly to produce the sex-associated difference in winter distribution. Under this hypothesis, as climate becomes harsher, the impact of male dominance over females becomes more severe, resulting in increasing withdrawal by females from locations settled by males and thus in clinal variation in sex ratio. Received 15 December 1978, accepted 27 February 1979. WE earlier reported that the sex ratio of Dark-eyed Junco (Junco hyemalis) populations wintering in the eastern United States varies dinally, with the percentage of females increasing progressively toward the south (Ketterson and Nolan 1976). Latitude and a set of climatic variables accounted for more than 93% of the variation in sex ratio found in the 1976 data set. That sample consisted of 616 museum specimens and 473 live-caught birds, the former group collected over many winters, the latter banded by us during four winters. Only cases from the period December through February were considered, and these were combined for analysis. Climatic variables were based on weather averaged over 50 yr (U.S. Weather Bureau 1932). Thus the pattern of geographic variation that we described was based on long-term sampling of winter populations and weather. This approach left a number of interesting questions unanswered, and we now state these and present our recent efforts to answer them. Objectives of this research were as follows: (1) to predict sex ratios of winter junco populations at previously unstudied sites using equations derived from our earlier work, to capture samples at those sites, and to compare observed and expected ratios, i.e. to learn whether our initial results had predictive value; (2) to sample sex ratios from several locations over more than 1 yr and thus to investigate the question of annual variation; (3) to compare sex ratios obtained in December alone with ratios obtained by pooling data from entire winters, as described above, and thus to learn whether the basic pattern of distribution is established by the settlement of juncos arriving in the winter range or by subsequent differential winter mortality according to sex; and (4) to determine the relative accuracy of various environmental variables in predicting sex ratio, i.e. to pursue investigation of the ultimate selective factor(s) responsible for geographic differences in winter sex ratio. We therefore obtained new field information from six sites and found published 532 The Auk 96: 532-536. July 1979 July 1979] Junco Sex Ratio Variation TABLE 1. Percentages of females in seven wintering junco populations, according to year. a 533 Sex ratio Location b Date N (% • •) X 21⁄2 Pullman, Washington (WA) Dec-Feb 1974/75 168 28.0 -(47øN, 117øW) Logan, Utah a (UT) Nov-Feb 1972/73, (42øN, 112øW) 1973/74 221 23.8 -Kalamazoo, Michigan (MI) Dec 1976 111 22.5 0.67 (42øN, 86øW) Dec 1977 105 17.1 Total 216 19.9 Portage, Ohio (OH) Dec-Feb 1975/76 46 37.0 0.30 (41øN, 84øW) Dec-Feb 1976/77 68 32.4 Dec 1977 22 36.4