Sunday, 19 August 2007

Researchers crack egg laying conundrum

The egg-laying habits and seemingly altruistic behaviour of fairy-wrens from Australia have finally been explained.

Studies have shown that Superb Fairy-wren (Malurus cyaneus) mothers that have helpers to raise their young lay smaller, less nourishing eggs and they gain from this by living longer and raising more young.

In many animal species, parents caring for their offspring are assisted by so-called ‘helpers'. In this type of cooperative breeding, some adults help others raise young instead of breeding themselves. 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 research team, led by the University of Cambridge and biologists from other universities in the UK, South Africa and Australia, published their findings in the magazine Science. 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 although the offspring who receive additional food do not appear to gain any advantage as a result. Until now scientists had always wondered who gained from this behaviour.

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

Source: Science magazine

Photo: Male Superb Fairy-wren by Dr Rebecca Kilner