Silkie the "SHH" gene, called the ZPA Regulatory Sequence (ZRS). This causes ectopic "SHH" expression in the anterior of the developing limb bud, leading to increased tissue growth and digits. While the feet of the Silkie display polydactyly, the wings have the standard tridactyly (three digit) arrangement. The Japanese Silkie initially develops additional digits in the wing as an embryo, but these are lost prior to hatching. The genetic cause behind Silkie polydactyly differs from those that cause polydactyly in the Dorking chicken breed, which is due to ectopic "FGF4" expression in the AER, with ectopic "SHH" a secondary effect. The
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