This quotation and the subsequent publication of the details of the General Adaptation Syndrome (Selye 1937) have become the basis for a vast number of studies on the subject of stress in animals. Despite the fact that the term stress seems imprecise because it means different things to different people, the broad concept has proved to be remarkably robust. In Selyean terminology, stress describes an animal's defense mechanisms, and thus a stress stimulus (stressor) is any situation that elicits defensive responses (Selye 1963). Since the environment is a composite of interacting stressors, a bird's success in coping with it depends on the severity of the stressor(s) and the bird's physiological ability to respond properly. In the broadest sense, the environment includes all combinations of conditions in which the bird lives-external (temperature, light, etc.) as well as internal (disease organisms, parasites). Significant changes in the environment signal regulatory processes to attempt to maintain or reestablish the equilibrium or homeostatic state. There are two general types of regulatory processes: specific and nonspecific. A particular condition will elicit a specific response. For example, when the external environment causes a bird's body temperature to rise, surface blood vessels dilate to permit greater blood flow to the skin for more rapid heat dissipation, and feathers are rearranged to reduce insulation. These are specific responses to the specific environmental stimulus of