ALTRICIAL	Birds which hatch helpless, unfeathered, and with eyes closed.
BARBULE	The part of the feather that branches off from the barb (which, in turn, is a branch from the main rachis)
CAMOUFLAGE	Protective coloration (often combined with posture and behavior) which helps make a bird difficult to detect in its environment
CLUTCH	The complete number of eggs laid by a single female and incubated together
CONE CELLS	Visual cells on the retina associated with color vision and visual acuity at high light levels
DRAG	The resistance caused by the body's forward motion in flight
ENDANGERED	Species (or other taxonomic units) whose population sizes have fallen to critically low levels and are therefore in immediate danger of extinction without human intervention
EXTINCT	A taxonomic unit (such as a species) in which no living individuals remain -- they are gone forever; the concept of extinction may also be applied t populations within species.
FOLLICLE	A specialized group of cells in the epidermis (skin) from which feathers grow
GENUS	A grouping of closely related species within a family; the genus name is the first part of a species' two-part scientific name.
GIZZARD	The muscular portion of the stomach which grinds up food
INCUBATION	The care of the clutch of eggs by an adult, usually involving the transfer of body heat to the eggs through the brood patch
LEK	A communal display area within which many males gather to attract mates
LIFT	The net upward force (counter to gravity) experienced during bird flight, created by the shape of the wing
MONOGAMOUS	A mating system involving a simple pair bond formed between one male and one female
ORDER	A major taxonomic grouping within a Class (such as Class Aves, or birds) uniting related families. Birds are divided into some 27 to 30 orders (the exact number depends on the viewpoint of the scientist)
OVARY	The female reproductive organ within which the reproductive cells (eggs) are formed; most birds have only a single developed ovary
POLYGAMOUS	A mating system in which a bird of one sex may have two or more mates of the opposite sex
PRECOCIAL	Birds which hatch in advanced state of development, with a covering of feathers, open eyes, and the ability to be active.
RACHIS	The central shaft of the contour (body and flight) feathers, from which the barbs originate
ROD CELLS	Visual cells on the retina that are sensitive in low light situations and are typical of birds that are active at night
SPECIES	A "kind" of bird, usually referring to a group of interbreeding natural populations which are reproductively isolated from other such groups; a species represents a unique evolutionary lineage
TESTES	The male reproductive organs within which the reproductive cells (sperm) are formed; birds possess paired testes
THERMALS	Rising currents of warm air (usually forming over ground rather than water); soaring birds make use of thermals for lift
UROPYGIAL GLAND	The "preen gland" which secretes an oil used for feather hygiene and waterproofing
UROPYGIAL GLAND	The "preen gland" which secretes an oil used for feather hygiene and waterproofing
VANE	The surface area of a contour feather, formed by all the collective side branches off the rachis (central shaft)