Breeding and Nonbreeding Season Mortality of Territorial Male Red-Winged Blackbirds (Agelaius phoeniceus)

Abstract

-The mortality of territorial male Red-winged Blackbirds (Agelaius phoeniceus) was determined in a banded population during the breeding seasons of 1973-1978. I used these data to test the hypothesis that sexual selection for characteristics that are advantageous in territory defense or mate acquisition, or both, occurs at the expense of survivorship. Observations of males that returned to the study area to reestablish territories, and of the tenures of these males on their territories, enabled me to partition annual mortality into components that estimate mortality during the and breeding seasons. The mean annual mortality of territorial male Red-wings was 52%, with 29% mortality occurring during the nonbreeding season (1 June to approximately 1 March) and 22% mortality occurring during the breeding season before 1 June. The effect of body size on survivorship was investigated by correlating male wing lengths measured in the third year of life with their ages at death. The correlation between wing length and survivorship was negative (-0.438) and statistically significant (one-tailed P < 0.05). I conclude that mortality during the breeding season is a potentially important selective force in this species, and that sexual selection may have occurred at the expense of survivorship. Sexual selection for large males appears to be opposed by survival selection for small males. Received 13 January 1986, accepted 15 July 1986. DARWIN (1859, 1871) viewed natural and sexual selection as separate but complementary mechanisms of evolution. According to Darwin, natural selection favors adaptations that increase survival, while sexual selection favors adaptations that increase the ability to acquire mates but decrease survival. In contrast, contemporary evolutionary biologists consider both selection for increased survival (survival selection) and selection for increased mating ability (sexual selection) as components of natural selection (see Mayr 1972, Selander 1972). Sexual selection, however, is still thought to occur at the cost of decreased survival (e.g. Selander 1965, Searcy 1979, O'Donald 1980, Lande 1981, Searcy and Yasukawa 1981, Kirkpatrick 1982). Thus, when sexual selection produces sexual dimorphism, the larger and more conspicuous sex is presumed to pay a cost in higher mortality (see Andersson 1982, Searcy and Yasukawa 1983, Payne 1984). Survival selection therefore would act to limit the effects of sexual selection because it would favor smaller, less conspicuous individuals of the sexually selected sex. Although the opposition of sexual and survival selection has been presumed since Darwin (1859), there have been relatively few attempts to document the reduced survival that should accompany sexual selection. The Red-winged Blackbird (Agelaius phoeniceus) is a species in which sexual dimorphism is thought to be the product of sexual selection (see Searcy and Yasukawa 1983). Male Redwings are larger and more conspicuous than are females (Nero 1956a, b; Orians and Christman 1968), and sexual selection is a potentially important evolutionary force because the variance in mating success is high (Payne 1979). However, although sexual selection is thought to act more strongly on male than on female Red-winged Blackbirds (Payne 1979), male and female Red-wing survivorship is virtually identical (Fankhauser 1971, Searcy and Yasukawa 1981). Thus, there is no evidence that males pay a cost in increased mortality. Furthermore, there is little evidence that larger male Red-winged Blackbirds suffer higher mortality than do smaller males (Searcy 1979, Johnson et al. 1980). These observations are inconsistent with the generally accepted view that sexual selection is primarily responsible for the sexual dimorphism in size, plumage, and behavior in Red-winged Blackbirds (see Searcy and Yasukawa 1983). I tested the hypothesis that sexual selection has favored characteristics that enhance a male's ability to acquire a territory or mates, but at the expense of his survival (Selander 1965, Searcy 56 The Auk 104: 56-62. January 1987 This content downloaded from 207.46.13.105 on Wed, 25 May 2016 05:23:12 UTC All use subject to http://about.jstor.org/terms January 1987] Mortality of Male Red-winged Blackbirds 57 1979). This sexual-selection hypothesis predicts that the mortality of males during the breeding season should represent a considerable portion of annual mortality because males pay a cost that results from their conspicuous and energetically expensive attempts to defend their territories and attract mates. I tested this prediction by partitioning the annual mortality of male Red-winged Blackbirds into components that estimate mortality in the breeding and seasons. The hypothesis also predicts that counter-balancing survival selection will favor small males. I tested this prediction by examining the relationship between size and survival of male Red-winged Blackbirds.