The birds-of-paradise are members of the family Paradisaeidae of the order Passeriformes. The majority of species are found in eastern Indonesia, Papua New Guinea, and eastern Australia. The family has 42 species in 15 genera. The members of this family are perhaps best known for the plumage of the males of the sexually dimorphic species (the majority), in particular the highly elongated and elaborate feathers extending from the beak, wings, tail or head. For the most part they are confined to dense rainforest habitat. The diet of all species is dominated by fruit and to a lesser extent arthropods. The birds-of-paradise have a variety of breeding systems, ranging from monogamy to lek-type polygamy.
A number of species are threatened by hunting and habitat loss.
For many years the birds-of-paradise were treated as being closely related to the bowerbirds. Today while both are treated as being part of the Australasian lineage Corvida, the two are now only thought to be distantly related. The closest evolutionary relatives of the birds-of-paradise are the crow and jay family Corvidae, the monarch flycatchers Monarchidae and the Australian mudnesters Struthideidae.
A 2009 study examining the mitochondrial DNA of all species to examine the relationships within the family and to its nearest relatives estimated that the family emerged 24 million years ago, older than previous estimates. The study identified five clades within the family, and placed the split between the first clade, which contains the monogamous manucodes and paradise-crow, and all the other birds-of-paradise, to be 10 million years ago. The second clade includes the parotias and the King of Saxony bird-of-paradise. The third clade provisionally contains a number of genera, Seleucidis, the Drepanornis sicklebills, Semioptera, Ptiloris and Lophorina, but support values for some of these is inclusions is low. The fourth clade includes the Epimachus sicklebills, Paradigalla and the astrapias. The final clade includes the Cicinnurus and the Paradisaea birds-of-paradise.
The exact limits of the family have been the subject of revision as well. The three species of satinbird (the genera Cnemophilus and Loboparadisea) were treated as a subfamily of the birds-of-paradise, Cnemophilinae. In spite of differences in the mouth, foot morphology and nesting habits they remained in the family until a 2000 study moved them to a separate family closer to the berrypeckers and longbills (Melanocharitidae). The same study found that the Macgregor’s bird-of-paradise was actually a member of the large Australasian honeyeater family. In addition to these three species, a number of systematically enigmatic species and genera have been considered potential members of this family. The two species in the genus Melampitta, also from New Guinea, have been linked with the birds-of-paradise, but their relationships remain uncertain, more recently being linked with the Australian mudnesters. The silktail of Fiji has been linked with the birds-of-paradise many times since its discovery, but never formally assigned to the family. Recent molecular evidence now places the species with the fantails.
Birds-of-paradise are closely related to the corvids. Birds-of-paradise range in size from the king bird-of-paradise at 50 g (1.8 oz) and 15 cm (5.9 in) to the curl-crested manucode at 44 cm (17 in) and 430 g (15 oz). The male black sicklebill, with its long tail, is the longest species at 110 cm (43 in). In most species, the tails of the males are larger and longer than the female, the differences ranging from slight to extreme. The wings are rounded and in some species structurally modified on the males in order to make sound. There is considerable variation in the family with regard to bill shape. Bills may be long and decurved, as in the sicklebills and riflebirds, or small and slim like the Astrapias. As with body size bill size varies between the sexes, although species where the females have larger bills than the male are more common, particularly in the insect eating species.
Plumage variation between the sexes is closely related to breeding system. The manucodes and paradise-crow, which are socially monogamous, are sexually monomorphic. So are the two species of Paradigalla, which are polygamous. All these species have generally black plumage with varying amounts of green and blue iridescence. The female plumage of the dimorphic species is typically drab to blend in with their habitat, unlike the bright attractive colors found on the males. Younger males of these species have female-like plumage, and sexual maturity takes a long time, with the full adult plumage not being obtained for up to seven years. This affords the younger males the protection from predators of more subdued colours, and also reduces hostility from adult males.