| Popis: |
Recent studies of typically monogamous passerine birds have suggested that the fitness benefits males derive by caring for their young may not be as great as was previously thought. This study was conducted to determine whether parental care by male dark-eyed juncos, Junco hyemalis, serves to increase either the quantity or quality of young that they produce. Over a 4-year period, males were caught at the time their eggs hatched, and the subsequent growth and survival of the young of unaided females and control pairs were compared. Broods raised by unaided females gained body mass more slowly and fledged at slightly lower mass than those raised by two parents. However, fledging mass was not correlated with survival to independence. There were no differences in tarsus growth between the two treatment groups. Entire brood loss to predators occurred as often among females without male help as it did among those with male help. However, partial brood loss was more common among female-only broods than among controls; this difference was largely attributable to higher rates of starvation and exposure in female-only broods. There appeared to be an interaction between growth and predation. Female-only broods that were below the median mass of combined treatment groups at 5 days of age were more likely than all other broods to experience partial or complete predation. Male impact on offspring survival varied with age of the offspring. When years were combined, males tended to increase survival during the first half of the nestling period, but their impact at the time of nest-leaving was minimal. In all years, from nest-leaving to independence (ca. 2 weeks), broods without male help survived only about half as well as did those with male help. Independent young raised by one parent were as likely to return the following spring as were young raised by two parents. Thus, paternal care benefits males by improving the survivorship of their fledglings, and may also act as a buffer against poor female parental quality and inclement weather. However, the magnitude of these benefits is such that bigamous males might achieve greater reproductive success than monogamous males. Various possible male strategies are discussed. |
