FACTORS LIMITING FECUNDITY OF CAPTIVE BROWN-HEADED COWBIRDS

We conducted two controlled experiments with captive Brown-headed Cowb irds (Molothrus ater) to test potential limiting factors for egg produ ction 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 o f females and compared their fecundity to that of controls. The fecund ity of females on the restricted calcium diet declined significantly a nd averaged 61% fewer eggs than the controls. To test the host nest av ailability hypothesis, we removed all nests from the pens of females a nd compared fecundity to that of controls. Fecundity of females depriv ed of host nests was not significantly different from controls. Result s of these experiments suggest that fecundity of Brown-headed Cowbirds can be limited by dietary calcium, but not by availability of host ne sts. 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 con trol females averaged 26.3 eggs, significantly greater than that of th e same females as yearlings. Three females laid more than 40 eggs duri ng the breeding season and one female laid 77 eggs, including an egg e ach day for 67 consecutive days, surpassing the previous record for nu mber of eggs produced in a single breeding season by a wild bird. Brow n-headed Cowbirds appear to be under intense selection pressure to pro duce eggs at a high rate, but the ability of wild females to acquire s ufficient dietary calcium to form egg shells may pose a constraint on fecundity.