MASS REGULATION IN JUVENILE STARLINGS - RESPONSE TO CHANGE IN FOOD AVAILABILITY DEPENDS ON INITIAL BODY-MASS

1. The adaptive mass-regulation hypothesis suggests that birds should respond to worsening or unpredictable foraging conditions by increasin g body mass and energetic reserves. However, previous work on adult Eu ropean Starlings, Sturnus vulgaris L., has revealed that the response to worsening foraging conditions varies according to the seasonal stat us of the birds. Photosensitive birds respond to decreased time availa ble to feed by increasing mass, whereas photorefractory birds regulate mass at a fixed level. Juvenile Starlings are in a state of photorefr actoriness until puberty. In this paper, we examine whether photorefra ctory juvenile Starlings respond to worsening foraging conditions by r egulating mass at a fixed level, like adults, or whether they increase in mass, in accordance with the adaptive regulation hypothesis. 2. Bi rds were divided into two treatment groups: control and food deprived. Control birds remained on food ad libitum throughout the experiment. Food-deprived birds had their food removed for 5 h every other day ove r a 6-week period. The effects of these manipulations on the resulting changes in body mass and fat reserves were examined. 3. Food-deprived birds increased fat reserves and body mass in comparison with control s. However, the response to food availability depended on the initial body mass of the birds. Birds with the lowest initial mass exhibited t he largest response to the manipulation. This may be because birds wit h low energetic reserves are more severely affected by changes in the availability of food. 4. Juvenile photorefractory Starlings show a res ponse to changes in food availability that is unlike the response of p hotorefractory adults, We suggest that the mechanism of responding to environmental change, in addition to being modulated seasonally, may b e age structured. We speculate that the function of this age effect ma y be related to the low competitive status of juvenile Starlings. This subdominant age-class may be displaced to less favourable feeding or roosting sites; as a result, they may have to respond flexibly to a ra pidly changing and unpredictable energetic environment.