Yellow-rumped Thornbills sometimes breed co-operatively, with a pair being assisted by one or two auxiliaries (helpers), which help to build the nest and feed the young. The nest is a large and untidy structure of grass and bark with two parts: an upper 'false' cup-shaped nest and a lower, domed, nest-chamber with a hooded entrance. The function of the false nest is not clearly understood, with many theories being put forward, such as: deterring predators or parasitic cuckoos, a roosting place for male or fledglings, a 'practice' nest for the helpers or as a 'displacement' activity for males. The nest is usually in the dense foliage of trees, near the end of branches or in vines or mistletoe. The female incubates the eggs alone, but is assisted by the male and any helpers with feeding and protecting the young.