Geographic Variation and Its Climatic Correlates in the Sex Ratio of Eastern‐Wintering Dark‐Eyed Juncos (Junco Hyemalis Hyemalis)

Abstract

The sex ratio of Dark—eyed Juncos wintering in the eastern and central United States and Canada varies clinally along a latitudinal gradient. The percentage of @V @V among both museum skins and live—caught birds is °70% in the south, 20% in the north. When abundance according to latitude is also considered, an average ♀ appears to winter farther south than an average ♂ and hence probably tends to migrate farther. Latitude alone is an excellent predictor of sex ratio (r 2 = 85%), and latitude plus 13 other measures of climate explain virtually all the variation (r 2 = 96.6%). Extreme measures of climate, as compared to mean measures, are equally predictive. Principal component analysis indicates that snowfall, temperature, and latitude are the most important climatic variables associated with sex ratio. Because ♂ ♂ average larger than ♀ ♀ and are concentrated northward, mean wing length increases with latitude and is significantly correlated with climatic measures that vary with latitude. Further, larger birds within each sex may select higher altitudes as wintering sites. Sex ratio does not vary measurably with date in wintering populations. Among possible explanations for clinal variation in sex ratio are sex—associated differences in (1) advantages of early arrival on the breeding or wintering grounds, (2) impacts of inter— and intrasexual competition, and (3) effects of low temperature and intermittent food availability. Comparison of ♂ ♂ and ♀ ♀ with respect to potential fasting endurance, a size—related metabolic parameter, indicates that at 0 degrees C an average ♂ should be able to fast 4% longer (1.6 h) than an average ♀ at standard metabolic rates. An extremely heavy 7 ♂ might endured fasting up to 29% (10.7 h) longer than a very light ♀. These differences may confer greater survival ability upon the ♂ at latitudes where snow cover can often preclude feeding.