Birds that eat seeds are more sociable than birds that eat insects — a trait that appears to influence social evolution in many animals
There are 118 species of weaverbirds (family: Ploceidae) and most of them live in sub-Saharan Africa. These sparrow-sized songbirds get their common name from the elaborate nests they construct. But despite their mad nest building skills, weaverbirds have evolved all sorts of different ways to make a living. For example, some live on open savannahs and eat seeds whilst others dwell in dense forests and mostly consume insects.
And therein lies the crux underlying the vast differences in these birds’ life histories.
“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”, said the study’s senior co-author, evolutionary biologist Tamás Székely, who is a professor in the Department of Biology & Biochemistry, and a member of the Evolution and Biodiversity research group at the University of Bath.
Furthermore, the abundance of food in the savannah allows females to feed an entire brood without any help from her mate, freeing males to pursue their own selfish interests.
“Flocking also lowers the risk of predation out in the open by providing them safety in numbers”, Professor Székely added.
But these life history traits come with a cost.
“[I]n 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”, Professor Székely explained. Because many savannah weaverbird species breed in colonies, males must compete for nesting sites and those who manage to win such sites are thus able to attract several females.
But forest-dwelling weaverbird species have their own cost-benefit scenarios that they must adapt to as well.
“On the other hand, forest-dwelling, insect-eating birds must search a wider area for food as insects are more widely distributed”, Professor Székely explained in a statement. “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.”
These observations are not new. Naturalists and (later) evolutionary biologists have been scratching their collective heads over the social organization of various species ever since Charles Darwin (PDF) first did so more than 150 years ago. This link between behavior, ecology and evolution — between the diet, habitat and social organization — of birds was originally investigated by British behavioral ecologist, John Crook, in his 1964 dissertation (ref). Crook’s studies, which focused on the weaverbirds, sought to understand the source of their tremendous social diversity.
Crook’s research proposed two key hypotheses to explain why some weaverbirds live in flocks both during the non-breeding season (flocking vs solitary foraging) and during the breeding season (colonial vs solitary breeding), and how, in turn, colonial vs solitary breeding impacts their mating systems and sexual selection.
Despite his lack of statistical analyses (which are almost de rigeur for any PhD in the sciences these days), Crook’s keen insights into the natural histories of the weaverbirds have affected the thinking of generations of behavioral ecologists regarding the influence of ecology upon the evolution of avian mating systems and other life history traits.
“The associations between diet, habitat and social behaviour in weavers have been suspected for decades, but this is the first time they have been proven by statistical analysis”, Professor Székely said about this study.
To do this work, Professor Székely and his collaborators conducted a variety of statistical analyses on weaverbird diet, habitat type, distribution, and social behavior and compared these results to the weaverbird phylogeny (family tree). Not only did these analyses confirm Crook’s original arguments, but they revealed a previously undetected relationship, suggesting there could be a direct link between spatial distribution of weaverbird species during the non-breeding season and their mating system.
This study also found that diet and habitat predicted sexual dimorphism — the difference in appearance between the sexes (read more about the biogeography of sexual dimorphism here). Sexual dimorphism is often correlated with polygamy, whereas monogamous males and females tend to look alike.
This study raises a number of interesting questions. First, how is flock size influenced by resource availability and distribution? How might colony size influence the associated costs and benefits of colonial nesting? And I’m particularly intrigued by what might be the outcome if the researchers dramatically changed weaverbird sex ratios in a given study area?
Additionally, considering how climate change is wreaking havoc over the entire planet, how are these extreme environmental changes are affecting weaverbirds’ lifestyles? What sorts adaptations are weaverbirds showing in response to the rapidly changing climactic variables they are confronted by?
Third, how relevant are Crook’s hypotheses for understanding how social organization is shaped in other bird species, and in other animals? Might these hypotheses even predict how human endeavors — businesses and social organizations and the like — respond to environmental pressures?
Zitan Song (宋紫檀), András Liker, Yang Liu (刘阳) and Tamás Székely (2022). Evolution of social organization: phylogenetic analyses of ecology and sexual selection in weavers, The American Naturalist | doi:10.1086/720270
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