Birds With Child-care Assistance Invest Less In Eggs

In many animal species, parents caring for their offspring are assisted by so-called 'helpers', adults that forego reproduction to help others raise young. Although parents decrease the amount of food they provide to offspring when helpers are present, the additional supply provided by the helpers more than compensates for this reduction. As a result, chicks fed by parents and helpers tend to receive more than those raised without helpers.

However, research has shown that these offspring who receive additional food do not appear to gain any advantage as a result. This has raised the question of who really gains from helping behaviour -- a question that has baffled scientists until now. The research on a common Australian bird species has provided a novel answer.

Researchers have discovered that in the 'superb fairy-wren'*, mothers benefit more from helping behaviour than their offspring. As helpers provide chicks with a significant amount of additional food, mothers can afford to skimp on nourishing their eggs. Females that are assisted by helpers were found to lay smaller eggs with disproportionately smaller yolks, thereby saving energy during egg laying. As a result, they live longer and breed more often than females with no helpers.

The scientists predict that this phenomenon occurs in other cooperative breeding bird species.

"Helper birds offer mothers a form of child-care", says Dr Rebecca Kilner from the University of Cambridge's Department of Zoology, one of the leaders of the research team that made this discovery. "In this species, mothers effectively steal the child-care from their current brood and spend this energy on producing more young in the future."

Superb fairy-wrens sometimes breed as pairs, and sometimes as pairs assisted by between one and four helpers. Helpers are always male and often sons from previous breeding. Females are drab brown in colour, but males moult into a striking blue plumage to breed, hence the species' superlative name.

The article 'Reduced Egg Investment Can Conceal Helper Effects in Cooperatively Breeding Birds' is scheduled for publication in the 17 August edition of Science.

The research was carried out by a team from the Universities of Cambridge and Sheffield in the UK and the Australian National University, and Macquarie and Wollongong Universities in Australia. It was funded by The Leverhulme Trust, The Royal Society and The Australian Research Council.

* Superb fairy-wrens Malurus cyaneus are small (10 g) garden birds, common to south-eastern Australia.