Every parent can use a little help now and then, and birds are no exception. Some species even use nannies to feed and care for chicks. These "daycare" babies don't seem to do any better than offspring raised by mom and dad alone do, however, and researchers have struggled to figure out how birds benefit from the assistance. A new study has cracked the mystery: The nannies apparently allow mother birds to save their strength so they can lay eggs later on.
In most bird species, males and females pair up to rear the brood. But about 3% of bird species are cooperative breeders: Only one female in a group lays eggs, and the rest of the adults help feed the chicks. Although several studies have shown that chicks get more to eat when helpers are present, little evidence indicates that they grow faster or have a higher survival rate. These findings have led some researchers to propose that the system benefits the helpers rather than the chicks, perhaps because the helpers receive reciprocal aid if they become breeders later on.
A team led by biologist Andrew Russell of the University of Sheffield, U.K., set out to evaluate the costs and benefits of cooperative breeding in the superb fairy-wren of southeastern Australia. This species can breed in pairs and in groups of about 6 to 12 birds, which allows a comparison between the two strategies. In a study population made up of 68 bird nests, the researchers found that chicks that were raised in groups received 19% more food than those fed by their parents alone. Yet those chicks were no larger. The secret, the team reports in the 17 August issue of Science , was that eggs laid by females in cooperative groups were 5% smaller than those laid by females who bred in pairs, and their yolks had 12% less lipids and 13% less protein. Thus, the chicks started out smaller but caught up because they were fed more.
The researchers surmise that by laying smaller eggs and getting outside help--thus reducing the amount of energy they invested in their offspring--mother superb fairy-wrens enhance their own survival and thus their long-term breeding potential. This conclusion is bolstered by the authors' analysis of data from a neighboring population of the same species, which revealed that females that bred in groups had a 30% lower mortality rate than those that bred in pairs.
Russell and his co-workers "have discovered a hidden reproductive strategy" in breeding females, says Jonathan Wright, a behavioral ecologist at the Norwegian University of Science and Technology in Trondheim. And Ashleigh Griffin, a behavioral ecologist at the University of Edinburgh, U.K., says that the report is "deeply satisfying" because it "clears up a mystery" about cooperative breeding. "There are several species of cooperatively breeding birds where helper numbers or helper effort seems to have negligible effect on offspring survival, and that just didn't make any sense."