REPRODUCTIVE ENHANCEMENT BY HELPERS AND AN EXPERIMENTAL INQUIRY INTO ITS MECHANISM IN THE BICOLORED WREN

As part of continuing studies of sociality in the wren genus Campylorhynchus we have been studying the bicolored wren-a facultatively cooperative breeder-for the past 6 years in the central Venezuelan savanna. Reproductive groups have ranged in size from 2 to 5. In one of our study populations, only about 15% of the groups contained helpers, and nearly all these contained only a single male helper. In an adjacent population, the majority of groups contained helpers, and more than half of these contained several helpers of either sex. Territory size is, on average, much smaller in the latter population. In these populations, the presence of a single helper is associated with a three-fold increase in reproductive success. Additional helpers are not associated with further reproductive enhancement. Enhancement is chiefly due to an increased proportion of nest starts that eventually produce independent juveniles. This reproductive enhancement is not merely an epiphenomenon resulting from the presence of helpers on territories which are superior for other reasons, such as greater resource availability or the quality of particular parents. It is also not a function of the mean or variance in nestling feeding rate. Predator exclusion experiments, in which certain nests were artificially protected from terrestrial predators, suggested that the mechanism of reproductive enhancement was heightened effectiveness of nest defense. Helpers are usually non-dispersers from the parental territory, and have always been found to be close relatives of the nestlings that they assist in rearing.