Recovery of hematocrit and fat deposits varies by cage size in food-restricted captive red crossbills (Loxia curvirostra).

Hematocrit-or the percent volume of red blood cells in whole blood-is thought to fluctuate adaptively in response to changing oxygen demands that occur during different life activities and in different environments. Because red blood cells are made from materials that can be limiting, however, it is thought that hematocrit may also reflect general body condition and access to resources. We tested the effect of hydration state, resource restriction (i.e., time available to forage), and activity (i.e., different cage sizes) on hematocrit in captive red crossbills (Loxia curvirostra). We found no evidence that a mild dehydration protocol impacts hematocrit and only weak support that mild food restriction impacts hematocrit. Food restriction did, however, reduce fat deposits and fat loss was more significant in birds that were also sampled for hematocrit. Furthermore, food-restricted birds housed in flight aviaries recovered hematocrit but not fat stores following repeated blood sampling, whereas birds housed in small cages lost additional hematocrit but mitigated fat loss following successive bleeds. Together these results suggest that different flight demands may determine response to blood loss during food restriction, potentially revealing a trade-off between fat storage and red blood cell development. Our results also demonstrate the need for scientists to carefully record hematocrit data and the time course across which multiple tubes of blood are collected to avoid confounding real patterns with variation generated by sampling protocol.