Blood plasma biochemistry and the effects of age, sex, and captivity in Short-toed Snake Eagles (Circaetus gallicus)

Physiology can either be a product of or affect the behavior and ecology of individual animals. Blood plasma chemistry contains many components that are indicative of several aspects of a bird's physiological state at the time of sampling. Plasma chemistry is routinely used in clinical settings. However, despite its potential application in ecological studies, few birds have published reference values for these parameters from wild populations. Here, we analyzed the blood plasma chemistry for 17 parameters in Short-toed Snake Eagles (Circaetus gallicus) captured in southern Spain. Most values did not differ between age classes and sexes. However, we found differences between our captive and wild populations of fledgling eagles. These differences arose primarily in parameters associated with nutritional condition, as we expected due to captive individuals being fed ad libitum prior to blood draws. We also compared urea levels and a body condition index with hatching date and found a much stronger relationship with urea than the body condition index. Our results demonstrate the necessity to obtain blood chemistry reference values from wild populations and not just captive individuals, and also illustrate how these parameters can complement other traditional data collected when handling birds in the field.