FECUNDITY AND EGG-LAYING PATTERNS OF CAPTIVE YEARLING BROWN-HEADED COWBIRDS

Yearling Brown-headed Cowbirds (Molothrus ater) were monitored in captivity to determine individual annual fecundity, patterns of egg-laying, and clutch size. Twenty-five juvenile females were captured in the wild, placed in separate pens, and provided with nests containing eggs and/or mock eggs throughout the breeding season. These females produced a total of 361 eggs over a 68 day breeding period from 16 May to 22 July. Individual fecundity of productive females (n = 22) ranged from 1-40 eggs and averaged 16.4 eggs. Individual laying rates during the laying period ranged from 0.18-0.91 eggs/day and averaged 0.56 eggs/day. Laying patterns were highly variable; one female laid an egg each day for 32 consecutive days. Laying patterns of most females were not significantly different from random and gave no indication that eggs were produced in clutches or sets. The results indicate that Brown-headed Cowbirds are indeterminant layers capable of daily egg production over extended periods and that individual females are capable of producing over 30 eggs in their first breeding season. Given an adequate supply of suitable host nests in the wild, these reproductive traits have clear adaptive significance for an obligate brood parasite.