Study: Vegetarian Weaver Birds are More Sociable than Their Insect-Eating Counterparts

by johnsmith

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

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

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

Weaver birds are members of Ploceidae, a family of 118 small passerine bird species.

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

Whilst some weaver species live on the savannah feeding on seeds, other species live in the forest and mostly dine on insects.

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

They observed that birds living in the open savannah tended to flock together, foraging in groups to help find the best sources of seeds.

The same birds also nested in large colonies and often had a polygamous breeding behavior, pairing with multiple mates during each season.

In contrast, the species living in the forest tended to be solitary foragers and nesters that did not flock together or live in colonies. These birds tended to be monogamous breeders with a single 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 became a textbook 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, the males often have more colorful and flamboyant plumage whereas in monogamous species the males and the females tend to look identical.

“For birds that feed on seeds in the open savannah, flocking together improves feeding efficiency because it makes it easier to locate patches where there are abundant seeds,” Professor Székely said.

“Flocking also lowers the risk of predation out in the open by providing them safety in numbers.”

“However, in open habitats such as the savannah there are limited nesting sites, meaning the birds live together in a colony and this often leads to polygamous breeding.”

“On the other hand, forest-dwelling, insect-eating birds must search a wider area for food as insects are more widely distributed.”

“The relatively safer, closed habitat of the forest provides lots of suitable nesting sites, so the birds don’t need to live close together.”

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

“The 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’ve also shown the complex links for the first time between food type, grouping behavior and mating systems using phylogenetic analysis in an unusually diverse group of songbirds.”

The team’s paper appears in The American Naturalist.


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

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