Roughly 300 billion birds, of at least 10,699 species, now inhabit the Earth. Birds are distinguished from other (living) vertebrates by their feathers, which are unique outgrowths of the skin. Feathers are filamentous, soft in texture, flexible, lightweight structures with diverse functions. Feathers are dead structures that must be replaced through molt. Feathers are essential for both temperature regulation and flight.
All birds have beaks, or bills, which are toothless and covered with a horny sheath. The avian gizzard is a large, strong, muscular structure used primarily for grinding and digesting tough food. Bird bones are typically lightweight structures, being spongy, strutted, and hollow. Perching birds possess a reversed, opposable first rear toe, or hallux, which enhances the ability of birds to grip a branch or prey. The perching birds (Passeriformes) include the majority of all bird species.
Birds are endothermic (warm-blooded) and maintain high body temperatures. Birds produce external eggs (no bird species bears live young). Most birds form monogamous pair bonds, some for life, but many engage in additional sexual liaisons. Birds have large, well-developed brains and are more intelligent than many mammals.
Birds, particularly the songbirds, have the greatest sound-producing capabilities of all vertebrates. The syrinx is the vocal organ of birds. Birds can navigate using the Earth’s magnetic field and celestial cues. The highly developed color vision of birds reaches into the near-ultraviolet range of the spectrum. The broad hearing range of various birds encompasses both infrasound and ultrasound.
The varied diets of birds include invertebrates, vertebrates (including carrion), fruits, nectar, seeds, buds, and leaves. The size, shape, and strength of the bill prescribe the potential diet. The upper mandible, or maxilla, is a flattened, hollow, bony cone reinforced internally by a complex system of bony struts called trabeculae. The lower mandible is composed of three bones fused together. Covering both jaws is a horny sheath, or rhamphotheca. Many birds can flex or bend the upper half of the bill at the nasofrontal hinge. Different modes of locomotion further expand the ecological opportunities of birds. There are specialized fliers, swimmers, runners, waders, climbers, and perchers.
Reproductive rate, adult life span, and age at maturity differ more than 10-fold among species.
A few species of birds are very broadly distributed, but most bird species and families are restricted, or endemic, to a specific, limited region of the planet. The six major faunal regions are the Nearctic, Palearctic, Neotropical, Afrotropical, Indomalayan, and Australasian.
—September 2020
—May 2021
Lance Jones 