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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.
Gape: see Heads.
Genus, genera: a genus is the taxonomic level above species, and contains one or often more species that are each others closest relatives.
Gleaning: taking insects, spiders and similar animal matter from the surface of leaves, branches, bark, etc., as opposed to catching them in the air.
Gloger's rule: a zoogeographic rule stating that animals living in a warmer, more humid environment tends to be darker than those living in cooler, dryer areas.
Gonys: see Beaks. Related expressions are gonys spot, gonys angle = gondyeal angle also explained in the link.
Gular: This relates to the throat area and in birds is usually relating to either a bare patch or sac of tissue connecting the lower mandible to the neck. The male Magnificent Frigatebird shows the latter to superb effect during courtship displays.
Gynandromorph: an individual bird which shows male characteristics on one side and female characteristics on the other side, normally explained from them having both ovary and testis present in the body. The pattern changes abruptly at the mid line.
 References G
Hallux: the name for the innermost toe (in humans the big toe) which in most birds is the one pointing backwards. see Legs and Feet
Homoplasy: is used to describe that two species are identical in some trait not because of common ancestry but because of convergent evolution or random genetic events. As example of convergent evolution, compare the mouth of some small, insect eating marsupials in Australia with shrews in Europe; they are fairly much identical because they are selected to efficiently solve the same problem, catching and eating insects.
Hybrid, Hybridization: a hybrid is the offspring of a mating between parents belonging to two different species or subspecies. A well known example is the mule, which is a hybrid between horse and donkey. Hybridization between distantly related species will most often result in infertile offspring, while more closely related species might result in offspring that seems fully fertile but still has selective disadvantages meaning that in the big picture, the two forms does stay distinct. On the other hand, hybridization between subspecies by definition would result in fertile offspring; if there is reduced fertility or selective disadvantage, then the two groups might be on the way to becoming full species. Examples of bird hybrids found reasonably frequently would be hybrids between ducks in genus Anas or in genus Aythya or geese including Canada Goose x geese of either genus Branta or Anser.
Immature (there are two usages of this term):
Iridescence: see Diffraction at D
Iris: see Heads
Irrupt, irruption, irruptive species: an irruption is the occurrence of larger number of birds of a species beyond their normal range, usually happening in irregular intervals and always with some years where no extra individuals are seen. Many species have been encountered in irruptions, but some well known examples are usually resident species of predators eating lemmings that after years with many lemmings and therefore large production of young can be found far south of their normal range, such as Snowy Owls and Northern Hawk-Owls. Other examples are Bohemian Waxwings, some species of lapwings and sandgrouse, and several different species of the dry interior Australia. Sometimes, an irruption is followed by birds staying and attempting breeding, but usually the species will again disappear as local breeding species.
 References I
Juvenile: (synonymous with Juvenal) the first plumage with real feathers, reached by molt from the downy plumage of a nestling or pullus. Strictly, this is a plumage that will be followed by either the first adult plumage or by the plumage of an immature; however, there seems to be great confusion, and many will use juvenile and immature as synonymous. The juvenile feathers often are different than the feathers of later plumages: firstly, they often push out the downs that they replace, and the down may remain attached to the tip of the juvenile feather for a while; secondly, the body feathers often are more fluffy and downy than later feathers; and thirdly, tail feathers in passerines often are more pointed then tail feathers of later plumages2. Erritzoe et al.1 proposes a change in usage so that a juvenile bird is any "young bird that is out of its nest and able to care for itself".
 References J
Kettle: a flock (usually) of soaring birds such as raptors (e.g., Broad-winged Hawk) that migrate more or less as a flock. The word describes the way such a flock will look while circling around each other in a thermal, something they do to gain height.
Leapfrog patterns: this is used to describe patterns where two populations within a species (or among closely related species) display one pattern of phenotype (visible characteristics) while an intervening population differs. Recent DNA results indicate that the distant, identical populations are not more closely related than the intervening one, but that the repeated patterns might be results of local selection forces.
Lek, lekking: lekking is the behavior where males display in groups hoping that a female will choose one of the displaying males to mate with. Lek can therefore mean such a group, but it can also mean the place where such behavior takes place (colloquially called mating arena). Ruff and Manakins are examples of birds known for their lekking, but lekking happens in several additional groups.
Leucism, leucistic: best used about an animal that has reduced pigment everywhere except bare parts, but where the normal pattern is still present in "shadow" form, or where the plumage is completely (and for the species abnormally) white but where bare parts are normally colored. (Some authorities also include "Partial albinism" within this term). Usage of this term in birds is complicated by the fact that bird have not one but two independent types of pigment based on melanin (usually further divided into eumelanin (black or grey), phaeomelanin (tan to brown) and erythromelanin (chestnut-reds)) and carotenoid (can be further divided into carotenes (orangey), xanthophylls (mainly yellows), and carotenoic acids (red)) molecules. Production of each main class and often of each type can be individually influenced by genetic changes, sometimes resulting in birds where one pigment is normal and another is missing or too strong. Some authorities therefore recommend using schizochroism with prefixes describing which pigment is changed and in which direction. See also "Albinism" and Melanism.
Lore: (more often used in plural, lores) see Heads
 References L