Validation of temperature-sensitive radio transmitters for measurement of body temperature in small animals

As part of a study on the core body temperature (T(b)) of desert birds, we purposed to use temperature-sensitive implantable radio transmitters. Because of the difficulty in recapturing these birds, we needed to know if these electronic devices held their calibration over the duration of normal battery life (21 days). Our initial calibration at 40 degrees C was consistently less than that of the manufacturer, although in some cases differences were small: deviations in temperature as predicted by the company's calibration and our own ranged from 0.1 to 1.7 degrees C. We tested for drift in calibration and found that for the first 9 days of operation, most radio transmitters deviated by less than 0.5 degrees C from the initial calibration. Differences between predicted and actual temperature became progressively larger with time, sometimes exceeding 2.0 degrees C. If possible, we suggest that transmitters be calibrated, implanted, and then after taking data on T(b) of the free-living bird, the transmitters should be recovered and recalibrated. If it is not possible to recapture birds, then our data suggest that T(b) predicted by pulse periods are reasonable up to 9-12 days after the initial calibration but data recorded beyond this time period are potentially suspect.