Vegetarian birds more sociable than insect eaters

Seed-eating weaver birds aggregate and nest in colonies more frequently than insect-eating species, suggests new research from an international team of scientists led by the University of Bath’s Milner Center for Evolution. For the first time, the study statistically supports an influential ecological hypothesis about social behavior first proposed 58 years ago.

Weaverbirds are a family of 118 species of songbirds that live primarily in sub-Saharan Africa and are so called because of their elaborate nest construction.

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

The researchers examined 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 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 research, for the first time, statistically supports the ecological hypotheses 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 he has influenced generations of behavioral ecologists.

The new study, moreover, 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.

Professor Tamás Székely, from the Milner Center for Evolution at the University of Bath, who initiated and led the study, said: “For seed-feeding birds in open savannah, herding improves the efficiency of food because it facilitates the location of areas where there are abundant seeds.

“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, meaning birds live together in a colony, often leading to polygamous breeding.

“On the other hand, forest and insectivorous birds have to forage in a wider area, as the insects are more widely distributed. The relatively safer and closed habitat of the forest provides many suitable nesting sites, so the birds do not need to live close to each other.

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

“The associations between diet, habitat and social behavior in weavers have been suspected for decades, but this is the first time that 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 study’s first author, Dr Zitan Song, a postdoctoral researcher in Professor Yang Liu’s group at Sun Yat-sen University in China, is now planning a follow-up study. This aims to test the generality of Crook’s theory by studying the associations between diet, habitat and social behavior in a wide range of bird species.

The research team intends to push the boundaries even further by testing the validity of Crook’s hypothesis in mammals, fish and insects. They will also complement these large-scale studies by testing aspects of mating system evolution that have emerged in recent years, such as the effects of sex ratio in the population.

The study, funded by the British Ornithologists’ Union, is a collaboration between the University of Bath (UK), Sun Yat-sen University (China) and the University of Pannonia (Hungary), and is published in American naturalist (DOI: 10.1086/720270).