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This section is aimed at explaining the bird and biology specific vocabulary you are likely to meet in other threads in Birdforum.
Malar: malar stripe, see Heads
Mandibles: the two parts of the bill/beak, if used in singular, most often as lower mandible; see also Beaks
Maxilla: another name for the upper mandible = the upper half of the bill.
Melanism, melanistic: an abnormality where there is darkening of all parts of a bird, both feathers and bare parts (bill and legs). The full version of melanism would produce a completely black bird. Eurasian Blackbird is not melanistic because it has a yellow eye ring and bill (and because the black plumage only occurs in the male anyway). The opposite of melanism is albinism. See also "Partial albinism" and "Leucism".
Migration: regular, usually seasonal movements that usually take a species between a breeding area and an area for passing a period of time that would be less hospitable in the breeding range. Such movements can be along a north-south direction in many parts of the world, but in the tropics there are plenty of examples where migration happens from higher breeding grounds to lower non-breeding areas, and a third possibility is regular movements driven by rainfall variation (a semi-desert that receives rainfall during a set time of the year can be a good breeding ground during and after that rainfall but inhospitable the rest of the year). See also Nomadism, nomadic
Migratory navigation: plenty of stories exist about long distance migrants that return to the same breeding territory for two or more years. How birds do that is not fully known, but one common idea is that they have a migration program (inherited or learned) and that one important aspect of their navigation is the use of a compass. Three different compasses seem to exist and be used in birds: magnetic compass, solar compass, and a stellar compass; if one of these dominate and calibrate the other compasses is an area of active research and discussion. The magnetic compass could be of a simple type that basically measures a north-south direction, or it could be a more advanced inclination compass that can measure how far below the horizon that the magnetic north is; it is still unknown how the birds sense this, but a location near or in the eyes for their sensory mechanism seems probable. The solar compass in addition to the obvious effects can rely on polarized light, because near sunset and sunrise (and to a lesser extent the rest of the day), there will be a band of increased light intensity going north-south over the sky if you are sensitive to polarized light, which at least some birds seems to be. The stellar compass is fairly simple in that the north star is a constant and the rest of the sky rotate relatively to that point (a similar argument can be made for the south pole of the night sky).
Monogamous: A mating system used by many birds where one individual has only one mate for at least one breeding season. There is a distinction between socially monogamous and genetically monogamous. In socially monogamous species, "cheating" is quite common i.e., that each litter is comprised of offspring fathered by more than one male while genetically monogamous species actually have one mother and one father for all ofspring in a breeding season. In temperate regions, it is quite common to see socially monogamous species, while genetically monogamous species are more common in the tropics. Many other mating systems exists, especially in the tropics but also to some extent in the temperate regions.
Monophyly, monophyletic: a group containing all members of a Clade (opposite is paraphyletic). In the illustration, an ancestral bird gave rise to species A, B, and C, so a group containing all three would be monoplyletic, but a group containing only A and C would be paraphyletic. At genus and higher levels of taxonomy, there is wide agreement that taxons should be monophyletic; at the species level, there is difference of opinions.
Monotypic: a taxon that is not subdivided. For example the Hoatzin is a monotypic species because there are no accepted division in subspecies. It is also the only member of its genus Opisthocomus which therefore is a monotypic genus. Opisthocomus is the only member of the monotypic family Opisthocomidae. The opposite is Polytypic.
Morph: usually used about discrete, reproducible color variants that occur within the same population of a species; also known as color phase. Examples are plenty, such as dark and pale forms of Booted Eagle, many of the Screech Owls of the Americas occurring in grey, brown, and red forms, or in the pale vs dark bellied forms of Parasitic Jaeger. In many cases, the designation is used even if a few intermediate birds are known.
Nail: see Beaks
Nest parasite: see Brood parasite
Nictating membrane: a third eye-lid that is a fully or partially transparent membrane which can be pulled across the eye from front to back. It tends to be fully transparent in diving birds, and they will use this to cover their eye when diving. Other birds may use this membrane to moisten the eye or to remove debris (to what degree such uses are assumed vs based on data is unclear). .
Nidicole, Nidicolous: A bird that is born naked and with closed eyes (so synonymous with Altricial). Nidicolous is the adjective of that.
Nidifuge, Nidifugous: nidifuge is a hatchling that is able to move around and most often feed itself (so synonymous with Precocial). Nidifugous is the adjective of that.
Nocturnal: a species that is active during the night. Many of these actually start their activity at dusk and may still be active at dawn (such as Barn Owl in the UK), while others are strictly nocturnal.
Nomadism, nomadic: birds of nomadic species make movements that are not regular (contra Migration) but are instead driven by irregular availability of food, for example following rare but strong rainfall such as occur in the Australian outback. Example species could be Princess Parrot and several others.
Ocellus, Ocelli: Eye-like spot(s) such as those found in Indian Peafowl.
Omnivorous: eats several different kinds of food, for example both fruits and invertebrates.
Orbital ring: a ring around the eye consisting of naked skin, see Beaks. Some birds have a ring of differently coloured feathers around the eye, this is the eye-ring.
Palmation: more or less the same as webbing; see Legs and Feet. Semipalmated really means "with half palmations".
Pamprodactyl: Having all four toes on each foot pointing forwards as in the swifts. see Legs and Feet
Parapatry, parapatric: two populations that are each others neighbors, or in other words occur on opposite sides of a common border, for example being separated by a river or some other habitat difference. Often used about closely related taxons that may even have hybrid zone at the border.
Paraphyly, paraphyletic: opposite of monophyletic, which see.
Partial Albinism: This is a term that some people use a lot and some claim should not be used at all. The best usage of "partial albinism" is to describe the bird that has completely normal colors in part of its plumage but one or more feather areas that are completely white. Bare parts will be normal. See also "Leucism", "Albinism".
Patagial mark: an area along the leading edge of the underside of the wings (parts of the underwing coverts) stretching from the body to the wrist which in some hawks (such as Red-tailed Hawk) will have a contrasting darker color.
Phylogeny: The evolutionary relationship among species (or other taxonomic units). A phylogenetic analysis can be based on morphology, plumage or DNA sequences, or any combination thereof; the results are often illustrated as a phylogenetic tree. See also Clade.
Pishing: attracting birds to you using sounds made using your lips and mouth. See more here.
Polygynous: In polygynous species the male tries to mate with more than one female during the breeding season. Male Bowerbirds for example build a bower to attract as many females as possible. The female then has to build and attend the nest alone.
Polytypic: the opposite of monotypic. For example, the Bananaquit is divided into forty-one subspecies according to the Clements checklist.
Post-ocular: mostly in connection with the words "spot" or "stripe": many hummingbirds have a white spot behind the eye or a stripe starting behind the eye continuing diagonally backwards-down.
Primaries: a group of flight feathers, see Wings which also explains "primary projection". The number of primaries differ among different groups of birds with gulls normally having 10, which some passerines only having 9.
Precocial: about a bird that is well developed at hatching, is able to thermoregulate and often are able to feed. The hatchlings of these birds are often able to run around soon after hatching and will most often leave the nest in less than 24 hours. See also Altricial for the opposite. Many birds hatch in a state between the extremes, so that there is a continuum rather than two clearly separated groups. See also Semi-altricial.
Pullus: original meaning seems to have been nestling, often used for any young bird that is still in downs. See also Juvenile in J
Qualitative trait: something that can be scored as present or absent. For example in different subspecies of Greater Antillean Oriole, the upper rump can be yellow or black.
Quantitative trait: something that will be scored as a number. This could be the length of the bill, width of the bill, or in an oriole, the percentage of the underside that is yellow vs black.
Range: another word for distribution.
Ranging: some (most?) birds have the ability to separate sounds on a much finer time-scale than humans. The theory about ranging is that they use this ability to discern the extent of echoes in the sound of a singing competitor (for example from trees in the surrounding area) and thereby are able to determine the distance to the singing bird without relying on the amplitude of the song; the amplitude will vary with the orientation of the singer.
Remiges: (singular: remex) flight feathers of the wing, see General Anatomy.
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Scapulars: the feathers of the shoulder region, see Wings.
Schizochroism: see discussion under Leucism.
Secondaries: a group of flight feathers, see Wings.
Semipalmated: "with half palmations", or in other words, with slight webbing between the toes.
Speciation in sympatry: for two species to arise in sympatry is a difficult process to prove. For this to happen, one current idea is that sexual selection would be combined with some ecologic differences for sympatric speciation to happen. One example is mentioned in Vidua.
Species: the most important unit in taxonomy, and perhaps therefore, many different definitions of the word species exists (called species concepts). A thorough discussion of the different species concepts is far beyond the scope of this dictionary. See this Birdforum thread for a short discussion and for references to several papers on the subject.
Speculum: a contrastingly colored area of the secondaries, see Wings. This designation is most commonly applied to dabbling ducks, but can also be used for parrots to mention one.
Subspecies: a geographically defined population that differs in some defined way from another population. If it is impossible to say where one population ends and the other starts (because there is a very wide gradual variation (cline)) then there really is one population and they are the same subspecies, even if the most distant birds are very different. If there, within 1100 miles is 500 miles with type A, 100 miles of gradual change, and 500 miles with type B, then you should have two subspecies. It will be obvious that there is a certain degree of personal interpretation as to when an area of mixture is narrow and stable (the two taxonomic units are different species), a little wider or unstable (the two taxa are subspecies) or a little wider yet (one subspecies). See this BirdForum thread for a discussion of the topic. In Opus, subspecies are currently listed based on Clements Checklist with occasional mention of differences of opinion in other checklists.
Sympatry, sympatric: about two populations that occur in the same or at least overlapping areas. See also Speciation in sympatry.
Synanthropic: about a species that is usually living in close proximity to humans. Synonymous with eusynanthropic, and opposite of exanthropic.
Syndactyl: Having the third and fourth toes on each foot joined for at least part of their length. This is found in the kingfisher family. see Legs and Feet
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