There are three unusual features in the reproductive biology of some wren species: multiple nest building, polygamy, and cooperative nesting. The building of superfluous nests occurs in many genera of wrens, but is most pronounced in the marsh-living genus Cistothorus. Male marsh wrens may build up to 20 nests in a season. Typically, nest construction is by the male, with the female apparently selecting the nest to be used for breeding and adding the lining. The sheer amount of energy that goes into building apparently surplus and useless nests suggests that it has a strong evolutionary advantage, but there is debate as to the actual nature of that advantage. Doubtless it is useful to have back-up nests that can be rapidly refurbished in the event of damage to the breeding nest, but after one or two back-ups that advantage must surely be played out. It has also been suggested that extra nests act as decoys to predators, or that the number of nests that a male makes is an indication of his reproductive vigor; indeed, if their surplus nests are experimentally removed, male marsh wrens have difficulty attracting mates. Surplus nests are also useful for roosting and may be constructed exclusively for that purpose. In the white-breasted wood wren, breeding nests are substantial and well concealed. Nests specifically built for roosting tend to be flimsy, situated higher up, and have less camouflage, since the ability to make a rapid exit on disturbance is apparently of more importance.
Most wren nests are domed structures with a side entrance, although nests built by some cavity-nesting species such as the northern house wren have no roof. In many species—for example in the genera Thryothorus and Uropsila—the nest is an elaborate, often a two-chambered, beautifully built structure; in others, for example, the song wren (Cyphorhinus aradus), it is tattered and untidy. Sometimes, as in the winter wren, nests are artfully concealed. By contrast, the cactus wren usually builds in savagely spiny cholla cacti with little effort at concealment. Many tropical wrens specifically nest in acacia trees that harbor aggressive colonies of symbiotic ants that attack any intruder, and the birds gain vicarious protection from predators such as monkeys. In the absence of such ants, some wren species deliberately site their nests next to pendant hornets' nests for the same purpose.
Clutch size in wrens varies from two in many tropical species to up to ten in temperate zones. Eggs are white, immaculate to heavily speckled, or rarely, blue. Incubation is by females alone, but young are fed by both sexes, except in polygamous species, where male help may be rare.
Polygamy is especially pronounced in the genus Cistotho-rus, reaching its peak with the marsh wren. In some populations of that species, more than half of the breeding males are bigamists, and a significant number are trigamists. The sex ratio among marsh wrens also seems to be skewed, with females outnumbering males almost two to one.
Cooperative breeding occurs in a number of species, but reaches its greatest development in the stripe-backed wren (Campylorhynchus nuchalis) of northern South America. There, groups of up to 14 birds will defend a territory, but only one dominant pair actually breeds. Nesting success and the rearing of two broods a year are both strongly correlated to group size; without at least two helpers a pair will not attempt a second brood. Birds in a group are usually blood relations, but after a year or so all females and some males disperse to other groups, thus minimizing incest.
Large gaps remain in human knowledge of wren breeding biology. For example, of the wrens found only south of the United States-Mexican border, the nest and eggs have never been described in about a third of species, and for a good many others available information is sparse.
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