Female Condition and Delayed Benefits to Males That Provide Parental Care: A Removal Study

In Dark-eyed Juncos (Junco hyemalis), we have shown that male participation in care of the young usually has a small effect on the number of nestlings that leave the nest and markedly improves survival of fledglings to the age of independence. Here we ask whether male participation also improves male reproductive success by enhancing female condition for future reproductive efforts. We captured males when their eggs hatched, then released one group immediately (controls) and held the other group (experimentals). The mates of experimental males were therefore deprived of help in rearing their broods. Removed males were quickly replaced by new males, which rarely fed the young of the experimental males but usually mated with the experimental females for later nesting attempts. We compared experimental (unaided) and control (aided) females for differences in the potential costs of their respective reproductive efforts. As measures, we used percent loss of mass during the nestling period and absolute mass at nest-leaving. Mass was regarded as a potential correlate of physical condition that might affect subsequent reproductive success. We also considered whether aided and unaided females were equally likely to attempt a subsequent brood and whether subsequent broods were produced equally rapidly and were of similar quality. When data were combined over years and across brood sizes, unaided females lost more mass and weighed less when their young left the nest. However, the differences were significant in only one year. Unaided females that raised large broods lost more mass than those that raised small broods, whereas brood size did not influence mass in aided females. Treatment groups did not differ in the probability of nesting again after producing fledglings. After nest failure, if females renested, the brood interval was 18% (1.3 days) longer for unaided females; after success and rearing of young to independence, the brood interval of unaided females was ca. 21% (3.4 days) longer. Neither difference was statistically significant. In both groups when first-brood nests succeeded, size of the brood was not correlated with the brood interval. Finally, neither number nor mean mass of eggs in the subsequent clutch differed between unaided and aided females. Even if the greater loss of mass and the somewhat longer brood intervals of unaided females can be interpreted as reflecting poorer physical condition, these apparently had little effect on future reproduct