Honest Advertising, Sexual Selection, Courtship Displays, and Body Condition of Polygynous Male Harriers

Abstract

In polygynous Northern Harriers (Circus cyaneus) aerial displays reflect male body condition, and displays are therefore difficult to fake. The most vigorously displaying males attract the largest harems, and such displays are an accurate measure of a male's potential to provision their mates (the single most important influence on female fecundity). Whether females preferentially pick intensely displaying males because they are in better condition or because vigorous displays are associated with higher provisioning rates is unknown, but evidence suggests that both condition and provisioning are important mate-choice cues. I conclude that sexually selected behavior in harriers is largely consistent with the honest-advertising hypothesis. FISHER'S (1930) model of "runaway" sexual selection has been challenged because models based on female choice do not take account of problems associated with the low heritability of characters chosen by females (e.g. Gustafsson 1986). This led Kodric-Brown and Brown (1984) to suggest that selection has favored the evolution of costly phenotypic traits, which reflect genotypic quality. Thus, rather than abstract epigamic traits alone influencing female choice (e.g. Darwin 1871, Andersson 1982), females may use cues such as tail length as "truthful" markers of genetic quality. Similarly, Anders-son (1986) suggested that if secondary sexual traits mirror an organism's nutritional status, and nutritional status reflects genotypi• quality, then no initial mating advantage is necessary for FisherJan sexual selection to evolve. There is essentially no difference between these hypotheses other than the emphasis that Kodric-Brown and Brown (1984) put on exaggerated physical traits such as antlers and nuptial col-oration and Andersson's (1986) emphasis on body condition and nutritional status as a reflection of genotypic quality. In lek species that defend no resources, viability differences may be advertised through body condition, vigorous displays, or even tail length (Andersson 1986). For resource-defending species, sexually selected behavior may be an advertisement of resource quality (Searcy 1979) and possibly body condition. Thus, females may prefer intensely singing or displaying males because of the direct resource benefits 303 that will enhance their lifetlme fitness (Searcy and Andersson 1986). Assessing the Kodric-Brown and Brown/An-dersson model for resource-defending species is possible by establishing correlations between display vigor and body condition, assessing the relative quality of controlled resources or male investment and correlating them with display vigor, and determining whether close associations exist between display vigor and harem size (polygynous species) or temporal differences in mate attraction (monogamous species). Although sexual displays such as song intensity or song rate are closely correlated with paternal feeding, superior territories, and paternal nest defense in Red-winged Blackbirds (Agelaius phoeniceus) and Stonechats (Torquata saxicola) (Greig-Smith 1982, Searcy and Yasukawa 1983), no such data exist for birds that lack acoustical displays. I present evidence that the most actively displaying male Northern Harriers (Cir-cus cyaneus) attract the largest harems, that such intersexual displays cannot be faked because they appear body-condition dependent, and that they accurately advertise at least a male's potential to provision his mates. STUDY AREA AND METHODS