Age Structure of Adult Brown-headed Cowbirds in Southwest Colorado

Abstract

We live-trapped and banded Brown-headed Cowbirds (Molothrus ater) in La Plata County, southwest Colorado, during the breeding seasons of 1992 to 1999. We captured a total of 1034 adult males and 386 adult females. Although we could not determine the exact ages of newly captured males older than one year (after second year; ASY) and of newly captured adult females, we identified the minimum possible age of each bird captured or recaptured each year. We found that most cowbirds of either sex were 1 or 2 years old, though we recaptured some older individuals. The percentage of recaptured females ≥2 years old increased through the study but appeared to stabilize during the last three years, implying that these older individuals constituted a quarter to a third of the population. The mean estimated minimum age of adult males was significantly greater than that of adult females in 1997 but not in 1998 or 1999. The ratio of yearling to older males varied significantly by year with yearlings becoming more common in the later years of study. Incorporating the age structure of a Brown-headed Cowbird population, especially of females, may increase the utility of various models concerning host populations subjected to cowbird brood parasitism, and this information is relevant to estimating the number of cowbird eggs expected to be laid in an area during a breeding season.