Reproduction and Social Organization of the Black-capped Donacobius (donacobius Atricapillus) in Southeastern Peru

--A color-banded population of Black-capped Donacobius (Donacobius atricapillus) on a Peruvian oxbow lake was studied during one breeding season and censused for 3 yr thereafter. Eighteen territories along a marshy shoreline were occupied by groups of 24 donacobius. Each group contained a mated pair and 0-2 helpers, apparently offspring produced during 1-2 preceding breeding seasons. Helpers participated in nest surveillance, territorial displays, and feeding nestlings. Displays occurred at territory boundaries and consisted of loud duetting by the pair or group. Breeding coincided with the rainy months, October through April, and peaked in November-January. Clutch size was always two. Egg and nestling periods each lasted 17-18 days. No correlation was demonstrable between group size and overall fledging success. Trios, however, accounted for all cases in which both young successfully fledged, whereas pairs never fledged more than one young. Annual survivorship of breeding adults averaged 71%. Cooperative breeding in donacobius is associated with limited opportunities for acquiring breeding territories, as in many other cooperative breeders. The relatively short period during which young serve as helpers (and the resulting small family sizes), however, suggests that at other times or places breeding territories are more readily available to young. Received 9 November 1983, accepted 19 April 1984. THœ Black-capped Donacobius (Donacobius atricapillus) is distributed widely in lowland tropical South America (Meyer de Schauensee 1970). Little has been published on the behavior and ecology of this aberrant monotypic species, which was recently reclassified as a wren (Troglodytidae) rather than as a mimid (Clench et al. 1982, A.O.U. 1983). Only Skutch (1968) has briefly described the nesting of donacobius in Venezuela. In this paper, we describe the social organization and reproductive attributes of a population of donacobius living along the shore of an oxbow lake in southeastern Peru. With thio report, the species joins the ever-growing list of birds exhibiting cooperative breeding. METHODS AND STUDY SITE All observations were made on Cocha Cashu (Fig. 1), an oxbow lake about 1 km from the Manu River in the Manu National Park, Dept. Madre de Dios, Peru (71ø19'W, 11ø51'S), at about 400 m elevation. The lake is surrounded by tall tropical moist forest (Holdridge 1967, see Terborgh et al. 1984 for detailed habitat descriptions and a complete checklist of birds at the site). On the shallower (western) side of the lake, most of the margin is lined with marsh grass 804 and sedge (primarily Panicurn grande and Scleria microcarpa) up to 2 m tall. Other vegetation on this shore includes scattered patches of Heliconia marginata up to 5 m tall, occasional figs (Ficus trigona and two unidentified species), and, at the extreme northern end, some overhanging limbs of other forest trees. The opposite (eastern) shore has little marsh but, instead, is lined with overhanging limbs of forest trees and