Baer's pochard females of Baer's pochard and the ferruginous duck are quite similar, but that holds true for the females of almost all "Aythya" species. Eclipse males resemble females, but retain the white eye. Formerly classified as a vulnerable species by the IUCN, recent research shows that its numbers are decreasing more and more rapidly. It was consequently uplisted to endangered status in 2008. In 2012, it was further uplisted to critically endangered. Between 1987 and 2007, there were only eleven reports of groups of 100+ birds in mainland China. A recent census by World Wildlife Fund (WWF) China of Hubei province
Bird-of-paradise lek-type mating system. Others, such as the "Cicinnurus" and "Parotia" species, have highly ritualised mating dances. Males are polygamous in the sexually dimorphic species, but monogamous in at least some of the monomorphic species. Hybridisation is frequent in these birds, suggesting the polygamous species of bird of paradise are very closely related despite being in different genera. Many hybrids have been described as new species, and doubt remains regarding whether some forms, such as Rothschild's lobe-billed bird of paradise, are valid. Birds-of-paradise build their nests from soft materials, such as leaves, ferns, and vine tendrils, typically placed in a tree