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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.
Albinism, albino: a bird or other animal completely lacking pigment in both feathers/hair/skin, eyes, legs, toes and nail, bills, and all other surfaces of the animal. White Monjita, Snow Goose, Swans, and other birds that are normally white are not albinos: they lack the pink eyes and bill/legs taking the color only from the blood running through the area, and instead have fully colored bare parts and sometimes even black in specific parts of the plumage. A true albino is a rarity, a result of some type of mutation inactivating the production of all types of pigment colors, melanins and carotenoids. The opposite of albinism is melanism. See also "Partial albinism" and "Leucism".
Allopatry, allopatric: word used to describe that two populations (often species or subspecies) have non-overlapping ranges.
Alternate plumage: usually synonymous with breeding plumage (nuptial plumage). In most non-tropical species, this plumage is worn during spring and summer, but in Anas ducks, northern hemisphere male ducks will wear this plumage during winter to late spring. Some species have only one annual molt and really show only one plumage year round; if so, they are going to be considered to be in basic plumage (which see) year round. In some birds, the change from basic to alternate plumage is the result of wearing off disguising feather tips to reveal inner more strongly patterned feather parts instead of partial or complete moult leading to alternate plumage.
Alula: see Wings
Anisodactyl: The arrangement of toes on a birds foot where the inner toe points backwards and the others point forwards. The majority of passerines have this type of foot, some of the non-passerines do too but not the majority. see Legs and Feet
Altricial: about birds that hatch as blind, without down, and generally helpless. Most passerines fall into this group. The opposite is Precocial, but in reality, 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.
asl: frequently used abbreviation for "above sea level" especially when describing elevation ranges of montane birds.
Auriculars: see Heads
Austral: about things pertaining to the southern hemisphere.
Bare parts: in bird descriptions, a collective designation for legs, bill, cere, and if applicable, of other bare skin areas such as bare skin around the eyes in for example Bare-crowned Antbird
Basic plumage: mostly synonymous with non-breeding plumage (non-nuptial plumage). In most non-tropical species this is the plumage worn during winters, but in Anas ducks, this is named the Eclipse plumage (which see) and is worn in summer and fall. Opposite is Alternate plumage.
Bill-tip organ: this is a structure found near the tip of the bill in several types of birds that forage by probing. This consists of pits in the bill surface which in the living bird is occupied by cells that sense pressure changes. The assumption is that this allows the bird to perform "remote touch" which means that it can detect movements of animals which the bird does not directly touch. Bird species known to have a "bill-tip organ" includes members of Ibisis, Shorebirds of the family Scolopacidae, and Kiwis. There is a suggestion that across these species, the bill-tip organ is more well developed among species foraging in wet habitats (water column or soft mud) than in species using a more terrestrial foraging.
Brood parasite: A species that lays its eggs in the nest of other species and leaves the work with raising the young to them. (Also known as nest parasite).
Bristles: see Heads
 References B
Cere: see Heads
Clinal variation, Cline: variation within one taxonomic unit (either species or subspecies) that is correlated with geography, so that for example birds living in colder climates are larger than members of the same species living in a warmer climate (many species, Bergmans's rule) or north-eastern birds are brighter colored than south-western (e.g., Chestnut Woodpecker). The important part of this concept is that the shift from one type to another is gradual, not abrupt.
Colony, colonialism: members of some species breed very close together with only the nest and its immediate surroundings defended against other members of the same species. One disadvantage of such a breeding system is that the members on average will have to fly far to collect sufficient food for the young. The ideas about what the possible advantages could be are many, but one is that being in a colony could help protect against predation of eggs and/or chicks. This seems to be borne out in some cases, for example in Royal Tern where the pairs nesting centrally in colonies seem to suffer fewer losses than those nesting in the periphery.
Congener, congeneric: that two or more species belongs to the same genus
Crepuscular: about a bird that is more active at dawn and dusk than at other times of the day or night. See also diurnal and nocturnal.
Crissum: combination of undertail coverts and the area around the vent
Cryptic: about plumage or general appearance, to denote a species or sex that is less easy to find. Sometimes called camouflaged.
Culmen: see Beak
CY = calender year: one of the systems used to tell the age of a bird. In this system, a bird is 1cy from the day it hatches until midnight, December 31, it is 2cy (second calender year) for the next 12 month, etc. In some ways this is more intuitive than a system like "first summer" (see this), but is also has some disadvantages: the switch in designation is not coupled to moult, so to give a description of a photo, one would have to know the date at which it was taken.
 References C
Diffraction: structural colors that are produced by the light being reflected in different directions depending on color or some being reflected while other colors of light is allowed to pass through the surface of a structure such as a feather. A feather producing colors due to diffraction does not contain any pigment and in low intensity, diffuse light will look black.
Dimorphic: most often used in the expression "sexually dimorphic" which means that the two sexes differ in plumage or whatever other character might be discussed. For example in hummingbirds, males often have colorful areas while females are more cryptic, and males can have elongated ornamental tail feathers which female generally lack or have in reduced form.
Diurnal: about a bird that is active during daytime. The activity pattern a bird shows at one time of the year might not hold true at other times of the year; for example, old world warblers have diurnal feeding habits but many species migrate at night.
Eclipse plumage: the non-breeding plumage of male dabbling ducks that looks very similar to female plumage. The eclipse plumage is often replaced by the next breeding plumage well before winter is over, so that the male duck can have attracted the attention of a female before breeding season starts. A similar switch into a non-breeding plumage of brightly colored birds are also known for other groups, including some shorebirds (Ruff and Phalaropes), Sunbirds, Fairywrens, and Weavers.
Endemic (there are two usages of this term):
Established population: a population that is self-maintaining, not relying on input from the outside to maintain itself. This expression is most often used in the context of a feral population that only becomes tickable if it becomes established (no longer requires continued addition of birds through released or escaped individuals).
Eusynanthropic: about a species that is usually living in close proximity to humans. Synonymous with synanthropic, and opposite of exanthropic.
Exanthropic: Opposite of synanthropic.
Extant: opposite of extinct.
Extinct: a species no longer found alive anywhere. It seems to be common practice to wait 50 years after last confirmed observation before bestowing this designation on a species. In some cases, a designation "extinct in the wild" is used when a captive population still exists; one could hope that would be basis for a future re-release into the wild.
Extirpated: a species no longer found in an area/region where it used to live, but one that is still found elsewhere.
Eye-bows: the arch-shaped often pale feather-tracts which on owls run from side of forehead down in front of each eye and ending at gape.
Eye-ring: The ring of feathers, often a different colour than the surrounding feathers, around the eye, often broken by the eye stripe. A prime example is that shown by White-eye sp. (Zosterops sp.).
 References E
Feral population, feral: a population that originated through release/escape of domesticated birds, such as the populations of Rose-ringed Parakeet outside their native range. A feral bird is one that originates in such a feral population.
First Adult Summer: This term does not indicate the age of a bird, it merely indicates that the bird is in its first adult summer plumage. In smaller birds, it is usually moulted into during the year after hatching (when the bird is in second CY), however with many large birds (for example gulls, eagles, albatrosses) this may be up to 3-5 years after hatching. Normally the bird moults into this plumage from first winter plumage. Also known as first breeding plumage, first alternate plumage, or first nuptial plumage. Some species are able to (or at least can attempt to) breed before they reach the full adult plumage.
First Summer (there are two usages of this term):
First Adult Winter: Not indicative of the bird's age but rather bird's first full set of adult winter feathers, usually reached during the year of hatching although with some birds it may be up to 2 -3 years before the bird moults into this plumage. This plumage is reached after moulting from juvenile plumage (but see remarks under First Winter, usage 2, below). Also known as first non-breeding plumage or first basic plumage
First Winter (there are two usages of this term):
Fynbos: a word used to describe areas with a scrub- and flower-rich habitat found only in the Western Cape region of South Africa. This is found on nutrient-poor soil in both lowland and especially mountain areas with winter rains in this region. Relatively few birds are found in Fynbos but the level of endemism among these is high. True Fynbo specialists are especially found in the higher elevation, while lowland Fynbos used to share birds with areas that are more nutrient rich but drier (renosterveld) but which today have largely been converted to agriculture.
 References F