Delayed plumage maturation and delayed reproductive investment in birds.
importance of testicular morphology in avian mating systems (Birkhead 1987; Møller 1988, 1991, 1994a, b; Westneat et al. 1990; Birkhead and Møller 1992; Briskie 1993), with few notable exceptions (Manning 1985, Sheldon 1994) investigators have largely ignored the eff ects of age-dependent variation in testis volume in studies of sperm competition and extrapair paternity. Yearling passerines are widely believed to be reproductively mature in terms of testicular size and production of sperm during their fi rst potential breeding season (Rohwer et al. 1980, Rohwer and Butcher 1988, A .—Passerine birds are favored models for studies of sperm competition and extrapair paternity, yet the intraspecifi c chronology of testicular maturation and its empirical and theoretical consequences in avian mating systems have been largely ignored. I analyzed age-dependent variation in testicular morphology in 25 breeding populations of the Blackthroated Blue Warbler (Dendroica caerulescens) distributed throughout its geographic range in eastern North America. Yearlings (fi rst breeding season) had signifi cantly smaller testes than older males (≥2 years). Latitude, altitude, and Julian date had negligible eff ects on testicular morphology when eff ects of core body size were controlled. Preparator eff ects had signifi cant infl uence on the estimation of testicular volume and asymmetry. Contrary to Møller’s hypothesis that the smaller testis compensates for defi ciencies in the larger, the volumes of the le and right testes were positively correlated in both yearlings and older males. Older males exhibited a higher degree of directional asymmetry because of the disproportionate enlargement of the le testis. These data suggest that testicular morphology and reproductive capacities of yearling passerines may not be equivalent to those of older males. In a broader context, these fi ndings demonstrate that age class should be factored into quantitative models of sperm competition in birds. Received 22 August 2003, accepted 16 January 2004.