The paper reviews hypotheses explaining the regulation of food intake in mammals that have addressed specific anatomical structures in the brain. An hypothesis, poikilostasis, is introduced to describe multiple, homeostatic states whereby the regulation of metabolism and feeding occur in birds. Examples are given for both wild and domestic avian species, illustrating dynamic shifts in homeostasis responsible for the changes in body weights that are seen during the course of an annual cycle or by a particular strain of bird. The following neural structures are reviewed as each has been shown to affect food intake in birds or in mammals: ventromedial hypothalamic nucleus (n.), lateral hypothalamic area, paraventricular hypothalamic n., n. tractus solitarius and area postrema, amygdala, parabrachial n., arcuate n. and bed n. of the stria terminalis. Two neural pathways are described which have been proposed to regulate feeding. The trigeminal sensorimotor pathway is the most complete neural pathway characterized for this behavior and encompasses the mechanics of pecking, grasping and mandibulating food particles from the tip of the bill to the back of the buccal cavity. A second pathway, the visceral forebrain system (VFS), affects feeding by regulating metabolism and the balance of the autonomic nervous system. Wild, migratory birds are shown to exhibit marked changes in body weight which are hypothesized to occur due to shifts in balance between the sympathetic and parasympathetic nervous systems. Domestic avian species, selected for a rapid growth rate, are shown to display a dominance of the parasympathetic nervous system. The VFS is the neural system proposed to effect poikilostasis by altering the steady state of the autonomic nervous system in aves and perhaps is applicable to other classes of vertebrates as well.