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The Fan-tailed Cuckoo is a slender cuckoo and the adult bird is easily identified by a yellow eye ring (slightly greenish in young birds), its generally dark slate-grey back and wings, becoming pale rufous below, with a boldly barred black and white under tail. Younger birds are duller and browner in colour.

As with most other species of Australian cuckoos, the Fan-tailed Cuckoo is a brood parasite; laying its eggs in the nests of other species of birds. Host species include flycatchers, fairy-wrens, scrubwrens and thornbills, particularly the Brown Thornbill, Acanthiza pusilla. A single egg is laid in the nest and one of the host's eggs removed. The young cuckoo generally hatches earlier than the host's eggs and proceeds to eject the other eggs or hatchlings. The seemingly unaware foster parents then rear the cuckoo chick.

Reference  http://www.birdsinbackyards.net/