Architectural traditions in the structures built by cooperative weaver birds.

Humans cooperate to build complex structures with culture-specific architectural styles. However, they are not the only animals to build complex structures nor to have culture. We show that social groups of white-browed sparrow weavers ( Plocepasser mahali ) build structures (nests for breeding and multiple single-occupant roosts for sleeping) that differ architecturally among groups. Morphological differences are consistent across years and are clear even among groups with territories a few meters apart. These repeatable differences are not explained by among-group variation in local weather conditions, bird size, tree height, or patterns of genetic relatedness. Architectural styles are also robust to the immigration of birds from other groups.