Social birds that forgo breeding to help raise the offspring of other group members are far more likely to care for their own close relatives than for more distant kin. This fact was found by a new study.
The study, which looked at a highly social species from outback Australia, the chestnut-crowned babbler, also found that these birds work much harder to care for their brothers and sisters than the young of less-related group members.
The findings provide new insights into understanding why some individuals cooperate with each other for a common good rather than pursuing their own selfish reproductive agenda.
"Cooperation is a major evolutionary puzzle," said Dr Lucy Browning from the University of New South Wales and the University of Cambridge, who led the study and is a post-doctoral researcher at the UNSW Arid Zone Research Station, at Fowlers Gap, in far-western NSW.
"One idea is that by helping relatives with whom they share DNA, they can pass on their genes indirectly, but testing this idea in birds and mammals has proved surprisingly difficult.
"An alternative theory is that such cooperation is actually selfish because in group-living species like babblers, individuals can increase their own welfare by helping to make their group larger, irrespective of how closely related they are. The fact that babblers preferentially help family members makes it seem likely that promoting the success of kin is the reason they cooperate," she added.
Babblers live in groups in which most members help to take care of young chicks in the nest, despite not being the parents themselves. But like any team activity, some individuals do the lion's share of all the work, while others do nothing at all.
"We wanted to get to the bottom of why some 'helpers' were so industrious while others were apparently so lazy," said Dr Browning.
"We found that when helpers are caring for their brothers and sisters, they feed them three times more often than when they are unrelated. In other words, they are much more 'helpful' when looking after family," she explained.
The study took place between 2006 and 2008, with birds being fitted with tiny radio transponders that were detected each time an individual visited the nest in order to feed the chicks.
The findings have been published in the journal Proceedings of the Royal Society B.