Weaverbird Nest Aggregation and Evolution of the Compound Nest

Abstract

Possible steps in the evolution of the compound nest are considered by comparing three different species of weaverbirds (Ploceidae) of the subfamily Plocepasserinae, the Sociable Weaver (Philetairus socius) of southern Africa, the Grey-capped Social Weaver (Pseudonigrita arnaudi) of East Africa, and the White-browed Sparrow Weaver (Plocepasser mahali) of East Africa. We develop a multiple factor theory of the origin of the compound nest recognizing that the relative importance of the different factors will vary with circumstances and in different places. Gregarious breeding is typical of many weaverbirds, and is favored by security from predation and by social stimulation of nest building and breeding. Security and gregariousness are enhanced by the roofed nests characteristic of all Ploceidae, often by nesting in thorny trees, and sometimes by nesting close to colonies of aggressive insects. Feeding in large flocks often well away from the colony tree is characteristic of socius and arnaudi, both of which often nest in large colonies, whereas mahali, which lives in small colonies, generally keeps close to its colony tree. Year-round residence in one tree is characteristic of the Plocepasserinae, in contrast to the Ploceinae, or true weavers. The former subfamily also more characteristically lives in arid climates, uses dry grasses for nest materials, and builds nests during both breeding and nonbreeding seasons. In the dry climate old nests may persist for many months. Aggregation of nests into masses with two or more nests in physical contact is common in the Plocepasserinae and is favored by the persistence of old nests through the dry season, by building of thick nests at any time of year, and sometimes by nesting in association with aggressive social insects. Some massing of nests in mahali and arnaudi is likely to occur by mere chance, especially in larger trees and colonies. Temperature regulation is related to size of nest mass and to body size in a reciprocal fashion. The Plocepasserinae sleep in their nests the year round; the larger mahali sleeps one bird to a nest while the two smaller species, arnaudi and socius, often sleep in groups of two to five birds to a nest. Group sleeping and consolidation of nests into large masses have been shown to have thermal advantages during the cool nights of the arid tropical and subtropical habitat. Consolidation of nests also helps solve the problem of any shortage of nest sites. A common roof, built on by all, is seen over nest masses of socius. This thick communal roof gives added thermal advantages, and we believe also enhances protection from some predators. The tendency of arnaudi to build new nests directly under and fused to old nests, suggests that such old nests may have been precursors of a communal roof in