A main classification division among birds is that between passerines and nonpasserines. The passerine birds (order: Passeriformes) are the perching birds, with feet specialized to grasp and to perch on tree branches. These are the more recently evolved birds (the most “advanced”) and include all the small land birds with which we are most familiar—flycatchers, robins, crows, wrens, warblers, blackbirds, finches, sparrows, and so on. The passerine order is the most diverse of the bird orders, including more than 50 percent of all bird species (about fifty-eight hundred species) and about half the bird families. A major subgroup within the passerines (containing about forty-five hundred species) is called the oscines, or songbirds: they all have a distinctive, advanced syrinx, the sound-producing organ in their respiratory passages. The oscines are responsible for most of the avian world’s more melodic vocalizations. The remainder of the globe’s birds—seabirds and shorebirds, ducks and geese, hawks and owls, parrots and woodpeckers, and a host of others—are nonpasserines, divided among the other twenty-eight orders. The nonpasserine family accounts in this book pertain to the ratite birds (Ostrich, Emu, etc.) through the woodpeckers; the passerine accounts are the ones that deal with the pittas through the New World blackbirds.
Nature (IUCN) Red List shows that 1,481 species (13.5% of 10,994 recognized extant species)
are currently threatened with global extinction. These include 798 classified as Vulnerable [(VU)
7%], 460 as Endangered [(EN) 4%], and 223 as Critically Endangered [(CR) 2%]. A further 52
species are considered to be Data Deficient [(DD) 0.5%], as there is insufficient information avail-
able to apply IUCN Red List criteria to assess their extinction risk. Population sizes of threatened
species span six orders of magnitude, from 1–7 mature individuals of Oahu Alauahio (Paroreomyza
maculata) to 12,800,000–47,600,000 mature individuals of European Turtle-dove (Streptopelia tur-
tur); however, 73% of threatened birds (1,088 species) are estimated to have fewer than 10,000
mature individuals, 40% (595 species) have fewer than 2,500 mature individuals, and 69 have fewer
than 50 mature individuals (37). Bird species are nonrandomly threatened across the avian tree of
life, with richness of threatened species disproportionately high among families such as parrots
(Psittaciformes), pheasants and allies (Phasianidae), albatrosses and allies (Procellariiformes), rails
(Rallidae), cranes (Gruidae), cracids (Cracidae), grebes (Podicipediformes), megapodes (Megapo-
didae), and pigeons (Columbiformes) (37). Once phylogeny is controlled for, extinction risk is
associated with greater body size, longer generation times, and lower fecundity (52).
More threatened bird species (1,278, 86.4%) are found in tropical than in temperate latitudes
(469, 31.7%) (Figure 1b), with hotspots for threatened species concentrated in the tropical An-
des, southeast Brazil, the eastern Himalayas, eastern Madagascar, and Southeast Asian islands (53).
However, a higher proportion of temperate-zone restricted species (202, 21.1%) are threatened
than tropical-restricted species (1,011, 16.7%). All countries and territories host at least one glob-
ally threatened bird species, and ten have more than 75, with Brazil and Indonesia heading the list
at 171 and 175, respectively. The majority of threatened species (817, 55%) are endemic to single
countries or territories, but some species have large ranges spanning many countries [e.g., 128 for
Saker Falcon (Falco cherrug)], and 4% of threatened species occur in more than 20 countries. Re-
stricted range species are more likely to be threatened, and there are 2,720 species with breeding/
nonbreeding ranges of <50,000 km2 (Figure 1c). Some threatened species are also migratory or
nomadic (239, 16%) and represent considerable transboundary conservation challenges. Ongoing
taxonomic refinement resulting in splitting of polyphyletic species has thus far not had a great
impact on the overall proportion of threatened species. Newly split species are on average signif-
icantly less threatened than species whose taxonomic status remained unchanged (54), although
this may change as land-use change intensifies in megadiverse tropical areas such as Amazonia
(55).
Repeated assessments of extinction risk for all birds since 1988 provide information on trends
in their status. The Red List Index (RLI) illustrates trends in survival probability (the inverse
of extinction risk) based on the number of species in each Red List category