Effects of known age on male paternity in a migratory songbird

Many avian studies have shown that reproductive performance improves with age, but little is known about how key components of male fitness, extrapair and within pair paternity, vary across life spans. We tested for age effects on male paternity in purple martins (Progne subis) using cross-sectional analyses of known-aged males (1–9 years old) and longitudinal analyses of individuals sampled in 2 successive years. Microsatellite analyses found that 137 of 297 (46%) nests contained extrapair offspring and 273 of 1235 (22%) offspring were extrapair. Using a subsample of unique known-aged males (n = 160), we found significant linear and nonlinear effects of male age on the number of within pair offspring and, to a lesser extent, on the number of extrapair offspring sired. Male genetic reproductive success increased with age to 3 years and then leveled off. In longitudinal comparisons of known age males sampled in successive years (n = 41), within pair offspring increased with age, even for males ≥2 years old. Paired comparisons (n = 74) found that extrapair sires were older than the males they cuckolded, and that first-year males were significantly underrepresented as extrapair sires given the known age distribution in the population. Poor genetic reproductive performance in younger males is likely constrained through male–male competition during mate guarding and female choice for older males.