Study: Vegetarian weaver birds are more sociable than their insect-eating counterparts

New research supports an influential ecological hypothesis about social behavior first proposed 58 years ago.

Village weavers (Ploceus cucullatus) and their nests. Photo credit: Chao Zhao.

Weaverbirds are part of the Ploceidae, a family of 118 small passerine species.

They live mainly in sub-Saharan Africa and are so called because of the elaborate construction of their nests.

While some species of weavers live in the savannah feeding on seeds, other species live in the forest and feed mainly on insects.

In the new research, Professor Tamás Székely from the Milner Center for Evolution at the University of Bath and his colleagues examined data collected from previously published studies of many weaver species to investigate the relationships between diet, l habitat and social behavior.

They observed that birds living in the open savannah tended to congregate, foraging in groups to help find the best seed sources.

The same birds also nested in large colonies and often exhibited polygamous breeding behavior, mating with multiple partners each season.

In contrast, forest-dwelling species tended to be solitary foragers and nesters that did not congregate or live in colonies. These birds tended to be monogamous breeders with only one mate per season.

The study, for the first time, statistically supports the ecological hypothesis of social evolution developed by a British ethologist, John Crook, who first proposed the link between diet, habitat and social behavior in 1964.

Crook’s study has become a classic example of ecological effects on mating systems, and it has influenced generations of behavioral ecologists.

The researchers also found that diet and habitat predicted sexual dimorphism – the difference in appearance between the sexes.

In polygamous species, males often have more colorful and flamboyant plumage while in monogamous species, males and females tend to look alike.

“For birds that feed on seeds in open savannah, grouping improves foraging efficiency because it makes it easier to locate patches where seeds are abundant,” Prof Székely said.

“Flocking also reduces the risk of predation in the open by providing them with security in numbers.”

“However, in open habitats such as savannah, nesting sites are limited, which means birds live together in a colony, often leading to polygamous breeding.”

“On the other hand, insect-eating forest birds have to forage in a wider area because insects are more widely distributed.”

“The relatively safer, closed habitat of the forest provides plenty of suitable nesting sites, so the birds don’t need to live in close proximity to each other.”

“This more solitary social system also means they are more likely to stay with the same mate during breeding season.”

“Associations between diet, habitat and social behavior in weavers have been suspected for decades, but this is the first time they have been proven by statistical analysis.”

“This study is particularly exciting because we have also shown for the first time the complex links between food type, grouping behavior and mating systems using phylogenetic analysis in an unusually diverse group of songbirds. “

The team document appears in The American Naturalist.

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Zitan song et al. Evolution of social organization: phylogenetic analyzes of ecology and sexual selection in weavers. The American Naturalist, in the press; doi: 10.1086/720270