![]() air sacs Thin-walled, transparent sacs extending from the mesobronchi or the lungs to different regions of the body they act as bellows to bring air into the body and store it until expiration. age-specific survival rate The proportion of individuals in a particular age group in a population that survive a particular interval of time-usually a year. age-specific fecundity The average birth rate for females in a particular age group in a population. age structure The relative proportions of individuals of different ages-usually noted for a given population. afterfeather A small feather that grows from the lower shaft of a contour feather and resembles the main feather but in miniature. Afrotropical region Zoogeographic region including Madagascar, southern Arabia, and all of Africa south of the Sahara Desert. African barbets A family (Lybiidae, 42 species) of small, colorful, stocky African birds with large, sometimes serrated, beaks they dig their nest cavities in trees, earthen banks, or termite nests. aerodynamic valve A vortex-like movement of air within the air tubes of each avian lung, at the junction between the mesobronchus and the first secondary bronchus it prevents the backflow of air into the mesobronchus by forcing the incoming air along the mesobronchus and into the posterior air sacs. aerodynamic power For birds, the total power required to match the various forms of drag and thereby maintain flight. advertising displays Displays performed by one sex (usually the male) to attract a mate of the opposite sex also called mate attraction displays. adrenal glands Small yellow or orange endocrine glands at the cranial end of each kidney they produce a variety of hormones (including adrenaline, steroids, and the sex hormones) that are involved with circulation, digestion, and reproduction. adoption In avian biology, the peaceful acquisition of a lone chick or chicks by a pair of adults other than the biological parents. adherent cup nest A cup nest made of mud or saliva that relies on chemical forces to hold it to a vertical surface built by many swifts, including the Edible-nest Swiftlets of Southeast Asia, whose nests are used in the Asian delicacy bird’s-nest soup. adaptive value The contribution of a trait to an individual’s ability to survive and reproduce. adaptive radiation The evolution, from a common ancestor, of a variety of different species adapted to different niches the species usually have different morphologies and behaviors. adaptive management A type of ecosystem management (see separate entry) in which managers continue to learn more about the ecosystem as they proceed, and continually modify their management techniques to incorporate the new information. adaptive (or acquired) immunity The components of the immune system that become tailored to respond to previously encountered pathogens, usually involving antibodies that recognize and attack specific antigens on those pathogens. adaptive Describes a trait that better promotes an individual’s fitness than does some alternative form of that characteristic. All kinds of traits-from subtle aspects of internal physiology to complicated avian behaviors-are potentially adaptations. adaptation A genetically controlled trait that increases an individual’s fitness relative to that of other individuals. acoustic nerve See vestibulocochlear nerve. acid rain Rainfall that is unnaturally acidic owing to atmospheric pollution. acetabulum At the hip joint, the hollow on the pelvic girdle into which the head of the femur fits. These changes allow the eye to focus on objects at different distances. accommodation The changes in the curvature of the lens (and cornea, in birds) of the eye brought about by the action of the ciliary muscles. acclimatization Changes in a bird’s physiology after it has been in a new environment for several days or weeks that allow it to function better in the new conditions. accessory nerve The eleventh cranial nerve it carries motor output to constrict the neck muscles. abiotic factors Features of the environment that result from non-living physical attributes such as temperature, topography, rainfall, altitude, etc. abiotic Nonliving includes both things that are dead (such as dead leaves) and those that have never been alive (for example, rocks). abducent nerve The sixth cranial nerve it stimulates a muscle of the eyeball and two skeletal muscles that move the nictitating membrane across the eyeball. A abdominal air sacs A pair of air sacs in the abdominal region of birds that may have connections into the bones of the pelvis and femur their position within the abdominal cavity may shift during the day to maintain the bird’s streamlined shape during digestion and egg laying.
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