The sociable weaver (Philetairus socius) is a species of bird in the weaver family, endemic to Southern Africa. It is the only species in its genus Philetairus. It is found in South Africa, Namibia, and Botswana, but its range is centered within the Northern Cape Province of South Africa. The species builds large, compound, community nests, a rarity among birds. These nests are perhaps the most spectacular structure built by any bird.
Taxonomy and systematics
The sociable weaver was first described by ornithologist John Latham in 1790. Alternative names include the common social weaver, common social-weaver, and social weaver. Formerly, four subspecies were recognised, but the species is now considered monotypic. The sociable weaver is the only extant species in the genus Philetairus.
Phylogeny
Based on a 2017 DNA-analysis, P. socius belongs to the group of sparrow weavers (subfamily Plocepasserinae) and is most related to the genus Pseudonigrita. These two genera are the sister group of the genus Plocepasser. The most basic genus of the subfamily is Sporopipes. Provided that genera are correct clades, the following tree expresses current insights.
Description
Measuring around in length, the sociable weaver has a black chin, black, barred flanks, and a scalloped back. It typically weighs , and sexes are indistinguishable.
Distribution and habitat
The species ranges across northwestern South Africa and southwest Botswana, and extending northwards across Namibia. It is strongly associated with the arid savannas characteristic of the southern Kalahari region. The presence of stiff grasses such as Stipagrostis ciliata (syn. Aristida ciliata) – an important nesting material – is an important determinant of its distribution. The taller grasses and the fire-prone nature of the Northern and Central Kalahari regions may be a factor for the absence of the bird in those regions. The area is semiarid with low and unpredictable rainfall occurring mostly in the summer. The population has not been quantified, but the birds are described as "common to abundant".
Behaviour and ecology
Breeding
In the southern range of the weaver's habitat, breeding may occur any time of the year and is closely linked to rainfall. In the northern range, a discrete breeding season between December and August has been noted. They may skip breeding during years when rainfall is low and a substantial number (sometimes over half) of birds in the colony may never breed in a given season. Under typical conditions, weavers raise up to four broods per breeding cycle. Sociable weavers are known to assist in the care of younger siblings and unrelated hatchlings, and nearly all pairs are assisted by helpers. A mating pair has been recorded as producing nine broods in a single season in response to repeated predation of its young. Unlike northern temperate passerine birds, which commence breeding within the first year of life, sociable weavers exhibit delayed-onset breeding, at sometimes up to two years of age.
Nesting
Sociable weavers construct permanent nests on trees and other tall objects. These nests are amongst the largest built by any bird, and are large enough to house over 100 pairs of birds, containing several generations at a time. The nests are highly structured and provide birds with a more advantageous temperature relative to the outside. The central chambers retain heat and are used for nighttime roosting. The outer rooms are used for daytime shade and maintain temperatures of inside while outside temperatures may range from .
The nests consist of separate chambers, each of which is occupied by a pair (sometimes with offspring) and used to roost and breed. Nests are built around large and sturdy structures such as acacia trees or sometimes even telephone poles. The trees generally used for nest-building are Acacia erioloba, Boscia albitrunca, and Aloidendron dichotomum. The birds at Etosha National Park also use Colophospermum mopane trees for nesting. Large nesting colonies can be active across many generations, sometimes over 199 years. The nest appears like a large haystack in the tree. If seen from below, entrances into the chambers may be seen, giving a honeycomb appearance. The entrances may be about wide and can be up to long. Sharp sticks may be placed to deter predators from entering.