The breeding biology of Rainbow Bee-eaters (Merops ornatus) was studied between 1997 and 2001 in southeast Queensland, Australia. Rainbow Bee-eaters are unusual in that they combine migration and coloniality with cooperative breeding. Birds migrate from islands north of Australia to build nests either solitarily or in colonies containing ≤50 active nests that are attended by socially monogamous pairs or trios (45% of nests). Nesting was essentially forsaken during very dry seasons, whereas hatching success was reduced in wet seasons. Incubation period varied in duration, with chicks emerging 22–31 days after incubation commenced. Eggs hatched either synchronously or, more commonly, asynchronously (88% of clutches), with each chick emerging usually 0–3 days after the previous hatchling in the nest. Asynchronous hatching typically led to a marked size hierarchy within the brood, facilitating starvation of 22% of nestlings that had avoided nest disasters, such as predation or flooding. Duration of the nestling period was also variable, with chicks requiring a further 24–36 days to fledge. Nest predation by native predators—monitor lizards (Varanus varius) and yellow-footed antechinus (Antechinus flavipes)—was relatively low, affecting 10% of nests. In contrast, predation by two introduced species—dingoes (Canis familiaris dingo) and, in particular, cane toads (Bufo marinus)—was extreme, terminating 49% of all nests.