It is extremely difficult to generalize about any of the behaviors or nesting habits of passerines, because as a group they are so diverse. Perching birds exhibit a bewildering array of plumages and colors derived from diverse keratin structures as well as ingested pigments, such as carotenoids (Gray, 1996). Many passerines, such as some Old World Flycatchers (Muscicapidae) and African Widowbirds (Viduinae) have extremely long tail feathers or highly modified plumes (Birds of Paradise: Paradisaeidae) used in courtship displays. Several groups such as the Wattlebirds of New Zealand (Callaeidae) and Honeyeaters (Meliphagidae) have fleshy, bright blue, red or yellow wattles on the face and neck. Perching birds build their nests generally out of sticks or grass on the ground, in trees, and in the case of Dippers (Cinclidae) in the banks of fast-flowing rivers. Many passerines migrate from their nesting grounds in the Nearctic and Palearctic to more equatorial regions, or from southern temperate regions north to the tropics. Parental care by both sexes is common in passerines, although in some highly dimorphic and predominantly lekking groups, such as manakins (Prum, 1994) and birds of paradise (Diamond, 1986), females alone provide for young and build the nest. Cooperative breeding, in which young birds delay breeding and assist other individuals (often parents) in raising young and defending the territory, is common in several passerine groups, such as Australian fairy wrens (Maluridae) and New World Jays (Corvidae; Brown, 1987; Edwards and Naeem, 1993). Some of the most elaborate singers in the bird world are passerines (Kroodsma and Miller, 1996). Some passerine birds are poisonous to the touch and are avoided as prey by indigenous peoples (Dumbacher et al., 1992).
The Passeriformes is the largest and most diverse commonly recognized clade of birds. The Passeriformes (or ‘passerine’ birds) are synonymous with what are commonly known as "perching birds"; this group also contains within it a major radiation commonly known as songbirds (oscine Passerines or Passeri). Of the 10,000 or so extant species of birds, over half (~5,300) are perching birds.
Perching birds have a worldwide distribution, with representatives on all continents except Antarctica, and reaching their greatest diversity in the tropics. Body sizes of passerines vary from about 1.4 kg in northern populations of Ravens (Corvus corax) to just a few grams. Perching birds include some of the most colorful and mysterious of all birds, such as birds of paradise from New Guinea and the bright orange Cock of the Rock from tropical South America. Because of their high diversity, generally small body size and relative ease of observation, collection and field study, perching birds have historically attracted the attention of a wide range of descriptive and experimental biologists, including systematists, behavioral ecologists, and evolutionary biologists.
Historically, it is generally agreed that the Passeriformes constitute a monophyletic group. Raikow (1982) established this monophyly in an explicitly phylogeneticcontext. He noted that Passeriformes possess a suite of distinguishing characteristics, including a unique sperm morphology, a distinctive morphology of the bony palate, a simple yet functionally diverse foot with three toes forward and one (the hallux) oriented backwards, and a distinctive fore- (wing) and hindlimb musculature. There are few if any species which pose problems for avian systematists as to whether they are or are not passerines. Most of the controversy lies in relationships within the clade.
The sister group of the Passeriformes is not so much hotly contested as it is poorly resolved by existing data sets. Traditionally, Passeriformes have been considered closely related to a large group known as the “higher non-Passerines”. These include a number of clades such as cuckoos (Cuculiformes), hornbills, kingfishers and related lineages (Coraciiformes), and woodpeckers and relatives (Piciformes). Many of these groups possess a zygodactyl foot, a condition in which two toes point forward and two point backward. The sister relationship of Passeriformes to woodpeckers, the hornbill group and allies is reflected in Joel Cracraft’s phylogenetic hypothesis for major groups of birds based on cladistic interpretation of morphological and molecular characters (Cracraft, 1988). However, in the other major classification bearing on the relationships of perching birds, that based on DNA-DNA hybridization, Passeriformes appear as the sister group to a large, diverse group containing pigeons and doves (Columbiformes), cranes and rails (Gruiformes) and storks (Ciconiiformes)! These latter three groups share few obvious morphological characteristics with Passeriformes. However, the DNA hybridization tree links Passeriformes with these groups at a very deep level in the tree, rendering this result tenuous. A recent study of nuclear DNA sequences by Hackett et al. (2008) finds Psittaciformes (parrots) to be the sister group of passerines, with Falconidae (falcons) also close. Clearly, more work on the sister-group relationship of Passeriformes is needed, since this relationship will be the basis of any study seeking to identify whether or not Passeriformes are a particularly diverse group (e.g., Nee et al. 1992).
Species diversity:The tradition of recognizing perching birds (Passeriformes) as the most diverse and rapidly radiating clade has been questioned because there are few obvious “key innovations” that should cause systematists to recognize Passeriformes over any other arbitrarily larger or smaller monophyletic group within birds (Raikow, 1986). One point that has been missed in debates on this issue is that the branch leading to the songbirds (oscines), a group comprising 80% of extant perching birds, is the longest internal branch on the DNA hybridization tree produced by Sibley and Ahlquist (1990). This branch has also been one of the few to be well resolved in applications of mtDNA sequences to higher level questions in birds, presumably because it is long. Given the large number of clades that will require names under phylogenetic taxonomy, perhaps the length of branches leading to particular clades should be one criterion whereby systematists decide which of the many clades to name.
Origin and biogeography of passerines:The temporal and geographic origin of passerine birds is obscure. Traditionally the group was thought to have originated in the Tertiary, at about the same time as extant orders of mammals. Some recent workers favor a later, Eocene origin (Feduccia, 1995; Wilson, 1989), but the DNA -DNA hybridization data again favors an earlier origin (Sibley and Ahlquist, 1990). Recently some of the oldest oscine fossils have been uncovered in Queensland, Australia (Boles, 1995); this and other paleobiogeographical data suggest that passerines may have in fact originated in the Southern hemisphere (Olson, 1989).
