A Possible Relationship between Reversed Sexual Size Dimorphism and Reduced Male Survivorship in the Ruby-Throated Hummingbird

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An examination of 28 years of banding data from Powdermill Nature Reserve in southwestern Pennsylvania showed the following trends in the sex ratio (female: male) of Ruby-throated Hummingbirds (Archilochus colubris): 1.1:1 for immature hummingbirds caught in late summer and fall; 1.4:1 for adults in spring (April-May); 3.1:1 for adults in summer (June-7 August); and 4.1:1 for adults in fall (8 August-October). Hypotheses to explain these increasingly female-biased sex ratios include differential migration, capture bias and differential mortality. Banding data from eight other stations in eastern North America provided no evidence of different fall migration routes for males and females. We detected no capture bias at Powdermill, with respect to mesh size of mist nets, net wariness or net avoidance ability, but there was seasonal variation in the number of overlapping captures of males and females at individual mist nets: spring (81.5%); summer (27.6%); fall (44.7%). A capture bias resulting from differences between the sexes in the use of habitats sampled by our mist nets has likely contributed to observed sex ratio variation. Differential mortality, with males suffering greater losses than females, probably also contributed to this variation. Estimated annual survivorship calculated from returns of banded hummingbirds to Powdermill was lower for males (0.294) than females (0.446), but this difference was not significant. The mean minimum known age for returning male hummingbirds at Powdermill was significantly less than that for females; furthermore, the oldest known age for a returning male (three years) was half that observed for a female. The lower inferred survivorship for males may be related to reversed sexual size dimorphism in this species. The body mass of female Ruby-throated Hummingbirds was significantly greater than that of males and did not vary between May and August, whereas males weighed significantly less in June and July than they did in May and August. Low mid-summer mass in males, coupled with increased metabolic demands during the breeding season, may lead to a fatal "energy crisis" in this sex during nocturnal fasting or periods of inclement weather.