A new study has revealed that desert birds lay larger eggs when they have more female helpers in their family group.
The researchers found that mothers increased the size of their eggs when they had more female helpers on hand to assist in feeding the nestlings.
The number of male helpers did not affect egg size, probably because male helpers feed chicks at substantially lower rates than female helpers.
The benefits of cooperation
CHARLY TRIBALLEAU/AFP via Getty Images
The study, published in the journal PLOS Biology, focused on white-browed sparrow weavers, a species of cooperatively breeding bird that lives in the harsh environment of the Kalahari Desert in South Africa.
Cooperatively breeding birds are those in which only a dominant pair breeds and their grown-up offspring, particularly females, help to feed nestlings. Nearly 10% of the world's birds have non-breeding helpers of this kind.
The researchers from the University of Exeter conducted a remarkable continuous decade-long field study of 40 family groups of sparrow weavers.
They measured the weight of the eggs and the amount of food provided by the helpers to the chicks.
They found that mothers increased the size of their eggs by about 5% when they had more female helpers on hand. This increase in egg size was equivalent to an extra day's worth of food for the chicks.
The researchers speculated that mothers increased their investment in offspring at the egg stage by lightening their workloads at the chick-feeding stage.
They found that mothers with more female helpers did indeed enjoy significantly lighter workloads at the chick-feeding stage, as female helpers fed the chicks more frequently than male helpers.
The adaptations of desert birds
The white-browed sparrow weaver is a predominantly brown, sparrow-sized bird that has a conspicuous broad white stripe from above the eye backwards, and two prominent white wing bars and wing edgings.
In flight, a large white patch on the rump and upper tail-coverts is visible. The male has a black bill, while the female has a horn-colored bill.
The white-browed sparrow weaver is found in greatest numbers in north-central southern Africa, where it inhabits dry regions with woodland or wooded grassland.
It is also found in eastern Africa, southern Angola, and northern Namibia. It feeds mostly on the ground for insects and seeds and makes messy nests in trees
The white-browed sparrow weaver is well adapted to the harsh conditions of the desert, where temperatures can vary widely and water is scarce.
It can survive without drinking water, obtaining enough moisture from its food and metabolic processes. It also has a high tolerance for heat, and can regulate its body temperature by panting, fluffing its feathers, or seeking shade
The white-browed sparrow weaver is also known for its complex and varied vocalizations, which include a high-pitched, metallic song that sounds like a computer-generated laser gun.
It uses its song to communicate with its group members, defend its territory, and attract mates. It can also mimic the calls of other birds, such as raptors, to warn or deceive its rivals
The study is one of the first to show that mothers in cooperatively breeding birds actually change the size of their egg according to their social environment.
The researchers suggested that this maternal strategy of increasing investment in offspring before birth when helped could be something that occurs more widely across cooperative species, including humans.
"Helpers feed offspring 'after' they hatch in birds, or are born in mammals, but our findings highlight that helpers could have hitherto unexplored beneficial effects on offspring even before they are born, by triggering an increase in maternal investment before birth," said Dr Andy Young, senior author on the study at the University of Exeter.
"This discovery has potential implications for other cooperative species too, including ourselves, because this maternal strategy of increasing investment in offspring before birth when helped, whether in the egg or in the womb, could be something that occurs more widely across cooperative species."
The researchers also noted that their findings could have implications for conservation, as they showed that the presence of helpers could enhance the reproductive success of endangered cooperatively breeding birds.
