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Mass regulation in juvenile Starlings: response to change in food availability depends on initial body mass
Author(s) -
WITTER M. S.,
SWADDLE J. P.
Publication year - 1997
Publication title -
functional ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.272
H-Index - 154
eISSN - 1365-2435
pISSN - 0269-8463
DOI - 10.1046/j.1365-2435.1997.00041.x
Subject(s) - sturnus , juvenile , foraging , biology , starling , zoology , ecology
1. The adaptive mass‐regulation hypothesis suggests that birds should respond to worsening or unpredictable foraging conditions by increasing body mass and energetic reserves. However, previous work on adult European Starlings, Sturnus vulgaris L., has revealed that the response to worsening foraging conditions varies according to the seasonal status of the birds. Photosensitive birds respond to decreased time available to feed by increasing mass, whereas photorefractory birds regulate mass at a fixed level. Juvenile Starlings are in a state of photorefractoriness until puberty. In this paper, we examine whether photorefractory juvenile Starlings respond to worsening foraging conditions by regulating mass at a fixed level, like adults, or whether they increase in mass, in accordance with the adaptive regulation hypothesis. 2. Birds 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 5h every other day over 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 controls. However, the response to food availability depended on the initial body mass of the birds. Birds with the lowest initial mass exhibited the largest response to the manipulation. This may be because birds with low energetic reserves are more severely affected by changes in the availability of food. 4. Juvenile photorefractory Starlings show a response to changes in food availability that is unlike the response of photorefractory adults. We suggest that the mechanism of responding to environmental change, in addition to being modulated seasonally, may be age structured. We speculate that the function of this age effect may 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 rapidly changing and unpredictable energetic environment.