FACTORS LIMITING FECUNDITY OF CAPTIVE BROWN-HEADED COWBIRDS

We conducted two controlled experiments with captive Brown-headed Cowbirds (Molothrus ater) to test potential limiting factors for egg production in this obligate brood parasite. Twenty-four wild-caught females were placed in separate pens during the breeding season and provided with crushed oyster shell and nests containing mock eggs. To test the calcium limitation hypothesis, we removed oyster shell from the diet of females and compared their fecundity to that of controls. The fecundity of females on the restricted calcium diet declined significantly and averaged 61% fewer eggs than the controls. To test the host nest availability hypothesis, we removed all nests from the pens of females and compared fecundity to that of controls. Fecundity of females deprived of host nests was not significantly different from controls. Results of these experiments suggest that fecundity of Brown-headed Cowbirds can be limited by dietary calcium, but not by availability of host nests. The 12 control females produced a total of 316 eggs over a 89-day breeding period from 25 April to 22 July. Individual fecundity of control females averaged 26.3 eggs, significantly greater than that of the same females as yearlings. Three females laid more than 40 eggs during the breeding season and one female laid 77 eggs, including an egg each day for 67 consecutive days, surpassing the previous record for number of eggs produced in a single breeding season by a wild bird. Brown-headed Cowbirds appear to be under intense selection pressure to produce eggs at a high rate, but the ability of wild females to acquire sufficient dietary calcium to form egg shells may pose a constraint on fecundity.