Darwin observed that the males of some species, such as birds of paradise, were very different from the females. Darwin explained such male-female differences in his theory of sexual selection in his book The Descent of Man.  Once the females begin to select males according to any particular characteristic, such as a long tail or a coloured crest, that characteristic is emphasized more and more in the males. Eventually all the males will have the characteristics that the females are sexually selecting for, as only those males can reproduce. This mechanism is powerful enough to create features that are strongly disadvantageous to the males in other ways. For example, some male birds of paradise have wing or tail streamers that are so long that they impede flight, while their brilliant colours may make the males more vulnerable to predators. In the extreme, sexual selection may drive species to extinction, as has been argued for the enormous horns of the male Irish elk, which may have made it difficult for mature males to move and feed. Different forms of sexual selection are possible, including rivalry among males, and selection of females by males.
The birds-of-paradise are members of the family Paradisaeidae of the order Passeriformes. The majority of species in this family are found on the island of New Guinea and its satellites, with a few species occurring in the Moluccas and eastern Australia. The family has forty species in 14 genera. The members of this family are perhaps best known for the plumage of the males of most species, in particular 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-based polygamy. The family is of cultural importance to the inhabitants of New Guinea. The trade in skins and feathers of the birds-of-paradise has been going on for two thousand years. The birds have been of considerable interest to Western collectors, ornithologists and writers as well. A number of species are threatened by hunting and habitat loss. Birds of paradise are generally crow-like in general body-form, and, indeed, are the brother group to the corvids (crows and jays). 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 on average bill size varies with reproduction(sex), although species where the females have larger bills than the male are more common, particularly in the insect eating species. For reasons of camouflage plumage of the females typically blends well with their habitat, unlike the bright attractive colors found on the males. 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 centre of bird-of-paradise diversity is the large island of New Guinea; all but two genera are found in New Guinea. The two that are not are the monotypic genera Lycocorax and Semiptera, both of which are endemic to the Moluccas, to the west of New Guinea. Of the riflebirds in the genus Ptiloris, two are endemic to the coastal forests of eastern Australia, one occurs in both Australia and New Guinea, and one is only found in New Guinea. The only other genus to have a species outside New Guinea is Manucodia, one representative of which is found in the extreme north of Queensland. The remaining species are restricted to New Guinea and some of the surrounding islands. Many species have highly restricted ranges, particularly a number of species with restricted habitat types such as mid-montane forest (like the Black Sicklebill) or island endemics (like the Wilson's Bird-of-paradise). The majority of birds-of-paradise live in tropical forests, including rainforest, swamps and moss forest, nearly all of them solitary tree dwellers. Several species have been recorded in coastal mangroves. The southernmost species, the Paradise Riflebird of Australia, lives in sub-tropical and temperate wet forests. As a group the manucodes are the most plastic in their habitat requirements, with in particular the Glossy-mantled Manucode inhabiting both forest and open savanna woodland. Mid-montane habitats are the most commonly occupied habitat, with thirty of the forty species occurring in the 1000Ц2000 m altitudinal band