Donacobius is an aberrant monotypic species that is widely distributed
in the lowlands of tropical South America (north to eastern Panama). It
is a unique and easily recognized blackish-and-buff bird of marshes or
grassy areas near water. Pairs occupy narrow territories along the water's
edge, vigorously displaying and dueting at territorial boundaries.
For many years Donacobius was considered a mimid, and went by the name of "Black-capped Mockingthrush." This changed in 1982 when a paper given at the 100th A.O.U. convention (but never published) declared that "the Black-capped Donacobius is a wren, not a mimid." Behavioral studies in Peru, where 18 groups of banded birds were followed for three years, supporting this transfer (Kiltie & Fitzpatrick 1984). Donacobius proved to be a cooperative breeder, with helpers at the nest in feeding young and territorial defense. This cooperative breeding behavior is more prevalent in wrens than in mimids; in addition, the vocal and display behaviors were unlike any mimid but rather like some Campylorhynchus wrens. Sibley & Ahlquist (1990) continued to include Donacobius within wren from DNA-DNA hybridization studies.
A recent molecular studies that sequenced nuclear and mitochondrial DNA (Barker 2004) showed that the assignment to wrens was conclusively excluded, as was assignment to any sister group or to mimids. Rather, the molecular evidence showed a potential relationship with Old World species, closest to white-eyes and prinias in the selected out-groups chosen. It is possible that Donacobius is the lone remnant of some long ago Old World sylvoid radiation into the New World. It may be a relict, as appears to be the case with Sapayoa. But it could also be a New World representative of an Old World family, like Wrentit Chamaea fasicata , the only remaining babbler in the New World.
Given this new evidence, some authorities now place it in incertae
sedis for the moment (e.g., Dickinson 2003). I don't have this luxury.
It seems quite plausible that this unique taxa might be elevated to family
status in the future. f it is related to some Old World lineage, it has
been long separated from these relatives. Elevation to family status here
is preliminary and tentative and could well change in the future, but world
birders would be wise to seek it out. Fortunately, it is not hard to find.
a dueting pair of Donacobius can be fascinating. "They perch a few inches
apart, one slightly above the other and usually facing the other way; they
bob their heads, jerkily wag their partially fanned tails asynchronously,
and call antiphonally, the upper bird giving a hard chrrr, and lower
bird a loud kweéa. Often the orange skin on the sides of
the neck is then exposed" (Ridgely & Tudor 1994). While antiphonal
dueting is present in some wrens, this description sounds a lot like the
gonoleks of central Africa. They are somewhat in behavior, though. Gonoleks
tend to be secretive in the reeds while Donacobius often sits up in the
Black-capped Donacobius is entirely resident. A study of social organization and breeding (Kiltie & Fitzpatrick 1984) found that territories were maintained year round; that when one of a pair died or disappeared, the other pair remained in the territory; and that the missing spot was quickly taken by a member of a nearby group (although it could be as far away as the other end of the lake). Survival rate was high and although nestlings suffer high mortality, once fledged, youngsters have high survival rates until they disperse from the natal group.
The nests are open cups in the reeds or grass; this is feature that is very unlike any wren. So, despite initial placement as mimids and then as wrens, Donacobius is a unique species. Fortunately it is widely distributed in Amazonia, and is particularly characteristic of oxbow lakes and floating vegetation in sluggish streams. There is more to learn about its ancestry and relationships, but it does seem appropriate to cover it as a monotypic family for the moment.
Photos: The photos of Black-capped DonacobiusDonacobius atricapillus were taken at Cocachoca, Tambopata Nature Reserve, Madre de Dios Province, Peru, in June 1987. All photos are © Don Roberson; all rights reserved.
Family Book: There is no "family book" per se but Ridgely & Tudor (1989) has a good summary, and details of breeding biology and social organization are in Kiltie & Fitzpatrick (1984). I look forward to the Handbook of the Birds of the World discussion, in whatever chapter they place it.
Barker, F.K. 2004. Monophyly and relationship of wrens (Aves: Troglodytidae): a congruence analysis of heterogeneous mitochondrial and nuclear DNA sequence data. Molecular Phylogenetics & Evolution 31: 486-504.
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