Unpredictable food availability induces metabolic and hormonal changes independent of food intake in a sedentary songbird

Environments often vary with regard to their temporal resource availability, but little is understood concerning how resource predictability impacts animals. The adaptive regulation hypothesis suggests that organisms act to conserve their current energetic state during periods of diminished food access and recuperate their energetic reserves (fat and muscle) during periods of greater food availability. In contrast, the chronic stress hypothesis suggests that variation in access to food can induce a prolonged stress response, resulting in maladaptive usage of energy reserves and increased behavioral activity. To distinguish between these hypotheses we compared the behavioral, hormonal and metabolic responses of captive curve-billed thrashers, Toxostoma curvirostre, fed varying amounts each day (variable group) with those of birds fed a constant amount every day (constant feeding group). Birds of both groups consumed, on average, a similar total amount of food during the course of the study, but birds in the variable feeding group lost mass and increased their circulating initial levels of the stress hormone corticosterone, showed evidence for increased secretion of a hypothalamic stress peptide, vasotocin, used greater amounts of fat and protein energy reserves, and were more behaviorally active than birds in the constant feeding group. Overall, these findings support the chronic stress hypothesis and suggest that birds such as thrashers may be particularly susceptible to the perception of unpredictable variation in food supplies independent of actual energetic constraints.