Any perching bird. All passerines belong to the largest order of birds, Passeriformes, and have feet specialized for holding onto a horizontal branch (perching). The passerine foot has three forward-directed toes and one backward-directed toe. Most passerines have moderately curved, sharp claws. Some ground-dwelling species (e.g., larks, pipits) have flatter, longer feet. Species that spend much time airborne (e.g., swallows) have small, weak feet. Species that cling and climb (e.g., nuthatches) have strong, sharp, curved claws. Passerines include about 4,000 species of oscines (songbirds; suborder Passeri, or Oscines) and 1,100 species of suboscines. Suboscines lack the syrinx of the songbirds or have only a poorly developed one, but some can utter complex vocalizations. All passerines are land birds, abundant worldwide except in Antarctica. Most are insectivorous solitary nesters that build a cup-shaped open nest.