NATURAL-SELECTION AND SEXUAL SIZE DIMORPHISM IN RED-WINGED BLACKBIRDS

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 direct ional selection favoring small males and that female size is maintaine d by stabilizing selection wherein extreme phenotypes experience highe r mortality. Museum specimens collected from Ontario over a 95-yr peri od were used to compare the sizes of males and females collected in fa ll 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 mor tality among larger males. Among adult (ASY) males, large individuals appeared to have higher survival than small individuals, although amon g 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 th an before winter. Our results do not support the hypothesis that body size in male red-winged blackbirds is limited by selective mortality o utside 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 evid ence showing higher adult male than female mortality in sexually dimor phic species. Such patterns could arise as a cost to males of sexual s election and yet provide no insight into how natural selection opposes sexual selection for increased male size.