The following is a glossary of common English language terms used in the description of birds—warm-blooded vertebrates of the class Aves and the only living dinosaurs. Birds, who have feathers and the ability to fly (except for the approximately 60 extant species of flightless birds), are toothless, have beaked jaws, lay hard-shelled eggs, and have a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Among other details such as size, proportions and shape, terms defining bird features developed and are used to describe features unique to the class—especially evolutionary adaptations that developed to aid flight. There are, for example, numerous terms describing the complex structural makeup of feathers (e.g., barbules, rachides and vanes); types of feathers (e.g., filoplume, pennaceous and plumulaceous feathers); and their growth and loss (e.g., colour morph, nuptial plumage and pterylosis). There are thousands of terms that are unique to the study of birds. This glossary makes no attempt to cover them all, concentrating on terms that might be found across descriptions of multiple bird species by bird enthusiasts and ornithologists. Though words that are not unique to birds are also covered, such as "back" or "belly," they are defined in relation to other unique features of external bird anatomy, sometimes called "topography." As a rule, this glossary does not contain individual entries on any of the approximately 11,000 recognized living individual bird species of the world.
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addled eggs Also, wind eggs; hypanema. Eggs that are not viable and will not hatch. See related: overbrooding.
afterfeather Any structure projecting from the shaft of the feather at the rim of the superior umbilicus (at the base of the vanes), but typically a small area of downy barbs growing in rows or as tufts. Entirely absent in some birds—notably from many members of the Columbidae family (pigeons and doves)—afterfeathers can significantly increase the insulative attributes of a bird's plumage.
allopreening
A form of social grooming among birds, in which one bird preens another or a pair does so mutually. At times it may be used to redirect or sublimate aggression, such as one bird assuming a solicitation posture to indicate its non-aggression and invite allopreening by the aggressive individual.
alternate plumage Also, nuptial plumage; breeding plumage. The plumage of birds during the courtship or breeding season. It results from a prealternate moult that many birds undergo just prior to the season. The alternate plumage is commonly brighter than the basic plumage, for the purposes of sexual display, but may also be cryptic, to hide incubating birds that might be vulnerable on the nest.
altricial
Also defined: semi-altricial; altricial-precocial spectrum. Young that, at hatching, have their eyes closed; are naked or only sparsely covered in down feathers (psilopaedic); are not fully able to regulate their body temperature (ectothermic); and are unable to walk or leave the nest for an extended period of time to join their parents in foraging activities (nidicolous), whom they rely on for food. The contrasting state is precocial young, which are born more or less with their eyes open, covered in down, homeothermic, able to leave the nest and ambulate and to participate in foraging. The young of many bird species do not precisely fit into either the precocial or altricial category, having some aspects of each and thus fall somewhere on an altricial-precocial spectrum. A defined intermediate state is termed semi-altricial, typified by young that, though born covered in down (ptilopaedic), are unable to leave the nest or walk and are reliant on their parents for food.
alula
Also, bastard wing; alular digit; alular quills. A small, freely-moving projection on the anterior edge of the wing of modern birds (and a few non-avian dinosaurs)—a bird's "thumb"—the word is Latin and means 'winglet'; it is the diminutive of ala, meaning 'wing'. Alula typically bear three to five small flight feathers, with the exact number depending on the species. The bastard wing normally lies flush against the anterior edge of the wing proper, but can be raised to function in similar manner to the slats of airplane wings that aid in lift by allowing a higher than normal angle of attack. By manipulating the alula structure to create a gap between it and the rest of the wing, a bird can avoid stalling when flying at low speeds or when landing. Feathers on the alular digit are not generally considered to be flight feathers in the strict sense; though they are asymmetrical, they lack the length and stiffness of most true flight feathers. Nevertheless, alula feathers are a distinct aid to slow flight.
anisodactylous Descriptive of tetradactyl (four-toed) birds in which the architecture of the foot consists of three toes projecting forward and one toe projecting backward (the hallux), such as in most passerine species.
anting
Also defined: passive anting. A self-anointing behaviour during which birds rub insects, usually ants and sometimes millipedes, on their feathers and skin. Anting birds may pick up the insects in their bill and rub them on their bodies, or may simply lie in an area with a high density of the insects and perform dust bathing-like movements. Insects used for anting secrete chemical liquids such as formic acid, which can act as insecticides, miticides, fungicides and bactericides. The practice may also act to supplement a bird's own preen oil. A third purpose may be to render the insects more palatable, by causing removal of distasteful compounds. More than 200 species of bird are known to ant. "Passive anting" refers to when birds simply position themselves so as to allow insects to crawl through their plumage.
apical spot A visible spot near the outer tip of a feather.
apterylae Singular: apteryla. Also, apteria. Regions of a bird's skin, between the pterylae (feather tracts), which are free of contour feathers; filoplumes and down may grow in these areas. See related: pterylosis.
aviculture
The captive breeding and keeping of birds.
axilla Also, axillar region; "underarm"; "armpit". The "armpit" of a bird, often hosting covert feathers called axillaries.
axillaries Also, axillary feathers; lower humeral coverts; hypopteron. Covert feathers found in the axillar region or "armpit" of a bird, which are typically long, stiff and white in colour.
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