Mike Webster records in the tropics.
In most bird species, young hatched in one year grow rapidly and then leave home to breed on their own within a year or so. But in cooperatively breeding species, young adult birds may stay at home for a year or more to help their parents, who can potentially produce more offspring thanks to these helpers. Usually just one sex acts as a helper, so Cornell University's Stephen Emlen hypothesized that it should benefit parents to produce more of the helping sex (typically males) than the non-helping sex (typically females).
However, as any human parent knows, grown young who stay home are often as much of a hindrance as a help. In particular, stay-at-home older young may compete with their parents and nestlings for food and other resources. In fact, during food shortages, "helpers" in one Australian species, the White-winged Chough, bring food to the nest, bend over as if feeding it to the young (in case the parents are watching), and then eat the food themselves. Thus, in some species or under some conditions, female birds may benefit by producing fewer of the helping sex, not more.
Claire Varian, a researcher now at the College of William & Mary, and I investigated how many sons and daughters were produced in another cooperative species that breeds in northern Australia, the Red-backed Fairywren. In this species, sons often stay home to help their parents raise more offspring, but daughters always disperse away from home to breed on their own, not competing with their parents' new families for food. Accordingly, if sons really help their parents, we'd expect females to produce more sons. If sons weren't so genuinely helpful, we'd expect more daughters. Through many hours of sitting in blinds watching adults bring food to the nest, we found that helper sons did indeed bring food to the nest and feed it to the young. However, the amount of food brought by these helpers was low compared with what the parents brought, and parents with helpers did not produce any more young on average than parents without helpers. Thus, the benefits of having a helper son seemed weak at best in this species, and we predicted that females should produce more daughters than sons.
Testing this prediction would have been difficult before DNA analysis, because young fairywrens lack sex-specific plumage. But when we analyzed small blood samples collected from nestlings, we learned that female fairywrens produced many more daughters than sons. This effect was most pronounced in females who already had a helper son—nearly three-quarters of the young produced by these females were daughters.
Our results for the Red-backed Fairywren echo those found in some other studies. For example, in Western Bluebirds studied by the Cornell Lab's Janis Dickinson, females who already had a son helping were more likely to produce daughters than sons. In the Seychelles Warbler, daughters stay home to help sons disperse. In this species, a single helping daughter significantly increases the reproductive success, but a second or third helping daughter does not add much more. Jan Komdeur and colleagues found that in this species females without a helper were particularly likely to produce a daughter, but females who already had a helping daughter were highly likely to produce a son.
Thus, in at least some cooperatively breeding birds, females appear to produce more of the non-helping (dispersive) sex in most situations, but produce more of the stay-at-home helping sex in those situations where the benefit of having a helper is particularly high. The mechanisms by which a female bird can control the sex of the eggs she produces remain a mystery, yet research from a number of cooperatively breeding birds is beginning to show us the complex ways that female birds can manipulate the composition of their families.
Mike Webster is the director of the Macaulay Library.