The Yellow-olive Flatbill (Tolmomyias sulphurescens) is a small insectivorous passerine inhabiting Neotropic forests. Its breeding biology is poorly known despite its abundance and conspicuousness. We describe the nesting biology of Yellow-olive Flatbills from Atlantic Forest fragments in Belo Horizonte County, Minas Gerais State, southeastern Brazil. Eighty nests were monitored every 3–5 days from August to January between 1995 and 2000. Active nests were found from mid-September through late December with a peak from mid October through late November. First clutches were usually laid during the first rains, but prior to the main peak in annual rainfall. The Yellow-olive Flatbill builds closed, pencile nests on tree branches along streams or roads, principally of dark fungal (Marasmius sp.) fibers. Clutch size ranged from two to four white eggs. Incubation was irregular and hatching was asynchronous. Incubation and nestling periods were 20 and 23 days, respectively. Nesting success across all 5 years was 29% (mean among years  =  31%, CI  =  25–37%), and nest predation was the main cause of nest failure (49%). Mayfield estimates of nest survival were low (mean  =  26%, CI  =  17–36%), and the probability of an egg to produce a fledgling was only 10%. Fledging success was 0.8 fledglings per breeding pair, and chicks fledged at 107% (CI  =  106–108%) of mean adult body mass. Our results do not support the purported pattern of long breeding seasons for tropical birds. The Yellow-olive Flatbill laid unusually large clutches, had lower nest survival, and greater fledgling productivity compared with other tropical passerines.

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