This landmark finding explains the long-standing mystery of why co-operation among birds is associated with unpredictable environments worldwide.
White-browed sparrow weavers live in the Kalahari Desert, where rainfall varies dramatically and food is scarce during dry spells.
The birds live in family groups, with a single breeding pair and up to ten non-breeding "helpers" that assist with chick feeding.
Explaining the evolution of such seemingly selfless behaviour has been a focus of evolutionary research for many years.
The new study, by a research team at the University of Exeter, shows that families with more helpers successfully raise more chicks in dry conditions.
Interestingly, these families are actually less successful in wet conditions than those with fewer helpers – meaning that co-operative helping does not improve overall breeding success, but instead reduces weather-driven variation in breeding success, which can be beneficial in other ways.
"We were really surprised – our findings suggest that the birds co-operate to cope with the unpredictability of their environment," said Dr Andrew Young, of the Centre for Ecology and Conservation on Exeter's Penryn Campus in Cornwall.
"This is an exciting discovery, because other studies have found that co-operative behaviour among birds is associated with unpredictable environments worldwide, especially those with variable rainfall.
