Meeting Abstract
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. However, birds housed in flight aviaries had higher hematocrit than those housed in small cages and those in aviaries that were also food limited invested in recovering hematocrit at the cost of fat stores following successive bleeds. In contrast, food-restricted birds housed in small cages lost additional hematocrit in an apparent attempt to conserve fat stores following successive bleeds. Together these results suggest a physiological trade-off between investing in fat storage or red blood cell development and reveals a potential prioritization based on activity demands when faced with a reduction of resources. 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.
