Sparrows lay larger eggs when they have help and support 

A new study from the University of Exeter has revealed an intriguing aspect of avian reproductive behavior, specifically in desert birds known as white-browed sparrow weavers. The researchers discovered that these birds lay larger eggs when more female helpers are present to assist in feeding the chicks. 

The study emphasizes the non-breeding female helpers’ influence on the mother’s egg size, contrasting with the negligible impact of male helpers who feed chicks less frequently.

More energy to invest 

Dr. Pablo Capilla-Lasheras, the lead author from the University of Exeter and now affiliated with the University of Glasgow, pointed towards a reduction in the maternal workload at the chick-feeding stage as a probable explanation for this phenomenon. 

“We don’t yet fully understand why helped mothers are laying heavier eggs, but our results point towards one likely explanation,” said Dr. Capilla-Lasheras. “Helpers may allow mothers to invest more in offspring at the egg stage by lightening maternal workloads at the chick-feeding stage.”

“Our findings support this, as mothers with more female helpers did indeed enjoy significantly lighter workloads at the chick-feeding stage.”

Cooperative species

This research is one of the first to show that cooperatively breeding birds actually change the size of their egg according to their social environment.

“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 study senior author Dr. Andy Young.

“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.”

Context of the study

The insights stem from a comprehensive ten-year field study involving 40 family groups of white-browed sparrow weavers at the Tswalu Kalahari Reserve in South Africa. 

The study’s context is particularly compelling given the harsh and unpredictable environment of the region, with cooperative breeding seen as a strategy to ensure the reliable rearing of chicks.

Sparrow weavers stand out for their impressive level of cooperation, with up to 10 helpers assisting a breeding pair. The helpers completely give up their own reproduction, remaining within their family as non-breeding adults for up to six years while helping to raise their parents’ young.

White-browed sparrow weavers

White-browed sparrow weavers (Plocepasser mahali) are fascinating birds native to the arid savannas and semi-deserts of eastern and southern Africa. These birds are noted for their distinct social structure and cooperative breeding behavior. Here are some key points about them:

Social structure 

They live in family groups where typically only one pair breeds, while others, usually their grown offspring, act as helpers.

Breeding pairs are assisted by up to ten helpers, which is quite a high number for cooperative breeding birds.

Helpers, particularly females, play a significant role in rearing chicks, which influences the breeding pair’s reproductive strategies.

Nesting and habitat

Their nests are large, community structures made of grass and twigs, with separate chambers for each pair. These nests can be used year-round, providing shelter from extreme weather and predators.

The birds inhabit regions where the climate is harsh and resources can be scarce, requiring a collaborative effort to survive and reproduce.

Diet

The sparrows’ diet mainly consists of insects, seeds, and grains, which they forage for on the ground or in low shrubbery. The need to find sufficient food for the chicks is a driving factor for the cooperative nature of their breeding.

Research and conservation

The species is of interest to researchers studying the evolution of social behavior and cooperative breeding in birds.

While currently not endangered, they provide valuable insights into the ecological balance of their habitats and the potential impacts of climate change on arid-zone birds.

The long-term study of these birds, like the one conducted in the Tswalu Kalahari Reserve, adds to our understanding of their complex social behavior and reproductive strategies, and how these are influenced by environmental factors.

The research was funded by the Biotechnology and Biological Sciences Research Council (BBSRC).

The study is published in the journal PLoS Biology.

Image Credit: Andrew Young

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