page_title: Glossary of bird terms
section_title: S
hierarchical_section_title: Glossary of bird terms / S
caption_attribution_description: Deutsch: Fänge eines Riesenseeadlers (Haliaeetus pelagicus) English: Talons of a Steller's Sea Eagle (Haliaeetus pelagicus)
context_page_description: The following is a glossary of common English language terms used in the description of birds—warm-blooded vertebrates of the class Aves, characterized by feathers, the ability to fly in all but the approximately 60 extant species of flightless birds, toothless, beaked jaws, the laying of hard-shelled eggs, 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; types of feathers; and their growth and loss.
  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".
context_section_description: scales 
  Specialized types: cancella; scutella; scutes. Also defined: reticulae; acrometatarsium / acrotarsium. The scales of birds are composed of keratin like their beaks and claws, and are found mainly on the toes and metatarsus, though they may appear further up on the ankle. They do not overlap significantly, except in the cases of kingfishers and woodpeckers. The scales and scutes of birds were originally thought to be homologous to those of reptiles and mammals; later research, however, suggests that scales in birds re-evolved after the evolution of feathers. Bird embryos begin development with smooth skin. On the feet, the corneum, or outermost layer of this skin may keratinize, thicken and form scales. Bird scales can be organized into:
  i) cancella—minute scales which are really just a thickening and hardening of the skin, crisscrossed with shallow grooves;
  ii) scutella—scales that are not quite as large as scutes, such as those found on the caudal, or hind part, of the chicken metatarsus; and 
  iii) scutes—the largest scales, usually on the anterior surface of the metatarsus and dorsal surface of the toes. The rows of scutes on the anterior of the metatarsus is sometimes termed an "acrometatarsium" or "acrotarsium".
  Reticulae, by contrast, are small, keeled scale-like structures covering the bottoms of bird feet, but are not true scales. Evolutionary developmental studies on reticulae found that they are composed entirely of alpha-keratin (whereas, true epidermal scales are composed of a mix of alpha and beta keratin). This, along with their unique structure, has led to the suggestion that reticulae are actually feather buds that were arrested early in development. See also: podotheca.
  scapulars 
  Also, humeral region. Feathers covering a bird's scapula "at the base of the dorsal wing".
  secondaries 
  Also, secondary feathers; secondary remiges. A type of remex flight feather, they are connected to the ulna. In some but not all bird species, the ligaments that bind secondaries to the bone connect to small, rounded projections that are called quill knobs. Secondary feathers remain close together in flight (they cannot be individually separated like the primaries can) and help to provide lift by creating the airfoil shape of the bird's wing. Secondaries tend to be shorter and broader than primaries, with blunter ends. They vary in number from six in hummingbirds, to as many as 40 in some species of albatross. In general, larger and longer-winged species have a larger number of secondaries. Birds in more than 40 non-passerine families are missing the fifth secondary feather on each wing; a state known as diastataxis.
  semiplume 
  Also, semiplume feather. A type of feather that has features straddling the differences between true down and contour feathers, such as by lacking cross-attachment between their barbs (i.e., without a flange–hooklet anchoring system), but having a central, full-length shaft as in contour feathers. They are usually found on the margins of the pennaceous feather tracts and, like down, provide insulative qualities and help fill out the plumage. Crest feathers are a type of semiplume.
  sexual dimorphism 
  The common phenomenon amongst birds in which males and females of the same species exhibit different characteristics beyond the differences in their sexual organs. It can manifest in size or plumage differences. Sexual size dimorphism varies among taxa with males typically being larger, though this is not always the case, i.e., in birds of prey, hummingbirds, and some species of flightless birds. Plumage dimorphism, in the form of ornamentation or colouration, also varies, though males are typically the more ornamented or brightly coloured sex. Such differences have been attributed to the unequal reproductive contributions of the sexes.
  shaft
  See entry for rachis.
  sides
  The topographical area between the breast and the back, stretching from the axilla (underarm) to the Iliac crest.
  song 
  A type of bird vocalization associated with courtship and mating, tending to be longer and more complex than a bir
caption: Scales on the talons of a Steller's sea eagle (Haliaeetus pelagicus)
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