Summary1.The Yellow-tniled Thornbill is gregarious and associates in flocks during the breeding and non-breeding periods.2.The nesting season is in the spring, October being the peak month. This is correlated with the rising temperature gradient.3.Both the male and the female assist in the construction of the nest. The same nest site may he used by a particular pair for several successive years, and only one nest may be built each season.4.The incubation and fledging periods are 18–20 and 17–19 days respectively.5.The female alone incubates. Both parents feed the nestlings and fledglings. Co-operative feeding of nestlings by progeny from previous broods commonly occurs.6.Up to four broods may be raised in a season.7.Breeding mortality data indicate that the Yellow-tailed Thornbill is a moderately successful breeder and that most young hatched in successful nests are fledged.8.The mean clutch size is 3·2±0·5. A seasonal trend in mean clutch size occurs.9.The species is a common host of the Golden Bronze-Cuckoo which apparently removes one of the thornbill eggs when depositing its own. Most cuckoo chicks that hatch are fledged.