Patterns of overwinter mortality in the sexually dimorphic red-winged blackbird (Agelaius phoeniceus)were examined to test the predictions of the sexual-selection hypothesis that male size is limited by directional selection favoring small males and that female size is maintained by stabilizing selection wherein extreme phenotypes experience higher mortality. Museum specimens collected from Ontario over a 95-yr period were used to compare the sizes of males and females collected in fall and spring. In a separate field study, body sizes of returning and nonreturning male and female red-winged blackbirds were compared over a 6-yr period. Overall, there was no evidence of higher overwinter mortality among larger males. Among adult (ASY) males, large individuals appeared to have higher survival than small individuals, although among subadult (SY) males, large size may have been disadvantageous. Weak evidence of stabilizing selection on female body size was found. Among adults, sexual size dimorphism seemed more pronounced after winter than before winter. Our results do not support the hypothesis that body size in male red-winged black- birds is limited by selective mortality outside the breeding season. It is possible that size selection occurs earlier in life, when males are still in the nest. Our results suggest that caution should be exercised when interpreting interspecific evidence showing higher adult male than female mortality in sexually dimorphic species. Such patterns could arise as a cost to males of sexual selection and yet provide no insight into how natural selection opposes sexual selection for increased male size.
Data were collected from 1985 to 1991 as part of a long-term study of red-winged blackbirds breeding near the Queen's University Biological Station, located 50 km north of Kingston, Ontario. Most ASY males that we captured were territory holders but some were either floaters caught while trespassing on territories or were of unknown status caught in a summer roost. However, an ASY male's status should not influence our analyses because size does not appear to influence territory ownership in this cohort (Eckert and Weatherhead 1987a; Weatherhead et al. 1987; Shutler and Weatherhead 1991). By contrast, most SY males were floaters, but a few (ca. 5%) held territories. All females were banded as breeding birds. Females were separated into two groups on the basis of having either dull or brightly colored epaulets (following Muma and Weatherhead 1989), which correspond closely to SY and ASY, respectively.