Experimental feeding affects the relationship between hematocrit and body mass in Spotless Starling (Sturnus unicolor) nestlings
Tóm tắt
Hematocrit, the proportion of blood volume occupied by packed red blood cells, is frequently used as an estimate of phenotypic condition. Some studies in birds, however, suggest that hematocrit might not always be a good estimate of condition. We tested the reliability of hematocrit as an estimate of condition by investigating the relationship between hematocrit and two other estimates of phenotypic condition (body mass controlled for body size and T-cell-mediated immune response) in nestlings of the Spotless Starling (Sturnus unicolor) under different environmental conditions. Half of each brood received an experimental food supplement while the other half was kept as a control. Hematocrit was positively related to relative body mass only in control nestlings whereas the relationship between hematocrit and cellular immune response was far from significant in both groups of nestlings. As expected, experimental food supply weakened the relationship between hematocrit and relative body mass, but this effect was not mediated by a decrease in the variation of phenotypic condition among nestlings. Instead, the effect of food supplementation was condition-dependent, reducing hematocrit in heavier than average nestlings, and increasing hematocrit in lighter than average nestlings. These results suggest that hematocrit might not be a reliable estimate of phenotypic condition under certain nutritional circumstances.
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