The bird species composition of 88 patches of monsoon rainforest in the Kakadu National Park area of the Northern Territory was examined in the period April-November 1987. The monsoon rainforest avifauna is a 'cut-and-paste' community, comprising species derived from wet tropical rainforests, from Eucalyptus open forests, from mangroves, from paperbark woodlands and habitat generalist species. Its composition may be seasonally unstable, and is re-arranged between monsoon rainforests of differing floristics or environment. For most bird species considered, the distribution was well associated with a division of monsoon rainforests into six groups: coastal, subcoastal, lowland springs, sandstone springs, escarpment and rock outcrop. Escarpment and rock outcrop rainforests were relatively depauperate. Monsoon rainforests are fragmented into small patches. The distribution of many characteristic rainforest birds (e.g. Orange-footed Scrub-fowl, Rainbow Pitta, Grey Whistler), bird species diversity and a Conservation Index were correlated with patch size. Bird species more typical of Eucalyptus open forests were abundant in patches most disturbed by buffalo, pigs, weeds and fire. No species were particularly associated with the edges of monsoon rainforests, but several open forest species occurred more commonly at edges than in the interior of monsoon rainforests (e.g. Barshouldered Dove, White-throated Honeyeater and Doublebarred Finch). Birds occurring in monsoon rainforests are probably highly vagile, a response mainly to the marked seasonality of the environment and the fragmented nature of monsoon rainforests.