It has been shown that small passerines can counteract variability of food resources by actively regulating their body reserves through an increase of mass. However, the effects of food predictability on body mass regulation and other body parameters, such as immune functions, in larger species have been little studied. To analyse the response of the Hooded Crow Corvus corone to food abundance and predictability, we performed three experiments with controlled food provisioning under laboratory conditions. Body mass, TOBEC (total body electrical conductivity) lean mass index, blood parameters and immune organ masses were measured at the beginning and end of a 15‐day period. In the first experiment, the food release was predictable (same amount each day) but the quantity of food delivered to five groups of birds varied (37, 75, 100, 150 or 300 g/day). Low food levels induced a greater decrease in mass accompanied by an increase in erythrocyte sedimentation rate. In the second experiment, the same average quantity of food (100 g) was supplied according to either predictable or unpredictable (random) schedules. In this case, the crows lost more mass, and their erythrocyte sedimentation rate increased when food was unpredictable. In the third experiment, the same average quantity of food (150 g) was supplied according to either a predictable schedule or two schedules with different levels of variability. The group with a low level of variability did not differ from the control, while the group with a highly variable feeding schedule lost more mass. In this group, the higher mass loss was associated with greater variation of the erythrocyte sedimentation rate and a reduced Phytohaemagglutinin index of immunocompetence and haematocrit value. The results of experiments 2 and 3 contrast with findings in other passerines, which increase their mass when food availability is unpredictable. It appears that a body mass decrease in the Hooded Crow can be induced by a reduction of either food abundance (exp. 1) or food predictability (exp. 2, 3), and it is accompanied by a worsening of health state.
Only recently have there been attempts to assess the short-term variations of indices of body conditions and immunocompetence in free-living populations, and to evaluate the presence of significant fluctuations of these indices between years. We compared body conditions and size of immunocompetent organs (spleen and bursa of Fabricius) in wild populations of hooded crows in NW Italy during three years. Differences in population density among sites did not influence any of the variables, while a great difference was found between adult and subadult birds. In the study period, adults did not show any year-related variation of mass, structural size, lean mass (as assessed by TOBEC electroconductivity index), and spleen mass. Instead subadults of both sexes showed differences among years in mass; subadult females had the lowest values of bursa and spleen mass in the year when mass was smallest. The results show that in natural populations between-year variation of body conditions may exist. Subadults are probably most sensitive to adversities producing short-term variations in body conditions.
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