This protocol allows the collection of quantitative data describing heat loss through avian feather coats. This type of data is not readily available in the field of avian ecology. This method is useful for comparative studies that detail the loss of heat through avian feather coats, and can be applied to any avian taxa and mammals.
Set up a constant temperature hot water bath. Place a sheet of clear acrylic glass over the surface of the water bath. And over the glass, place a piece of one inch thick foam core board with a 0.5 inch circular hole.
Attach a thermal camera to a tripod directly above the setup at the camera's minimum focusing distance. To calibrate the camera, begin by clicking the override camera file, then set the distance to the minimum focusing distance of the camera. After setting the emissivity to one in the thermal camera software, place aluminum foil over the foam and capture a thermal image.
The temperature at the surface of the aluminum foil is the reflected temperature. Enter this number in the camera calibration software and set the emissivity to 0.95. Then enter the atmospheric temperature and relative humidity.
Set the hot water bath to the target temperature. In the thermal camera software, draw a circle or ellipse over the hole in the foam. This ensures that the correct area is measured when the flat skin is placed over the hole.
Place the flat skin specimen on the foam core with the area of interest over the hole. To position the skin correctly, gently touch the feathers in the target measuring area with one fingertip. The residual heat from the fingertip will briefly remain visible on the thermal image.
Move the skin specimen until the target measuring area is within the circle drawn in the software. The placement of the skin over the heat source has a large effect on the repeatability of data values. This step should be practiced before data collection.
Wait 15 minutes to allow the skin specimen to acclimatize to the heat source. If measurements are made too early, the temperature value at the surface of the feather coat will be too low. Take a thermal image of the flat skin, then change the emissivity to match the emissivity of the acrylic glass.
Remove the skin from the foam and immediately capture a thermal image of the acrylic glass. Then repeat the entire process. First, open the image of the acrylic glass.
After ensuring that the emissivity is set to the emissivity of the acrylic and that the atmospheric temperature and relative humidity match the conditions in the lab at the time the image was captured, record the temperature value from the center of the circle. Next, open the thermal image of the flat skin. Set the emissivity to 0.95, the emissivity of the feathers.
Record the temperature value at the center of the circle. Then repeat this process for all pairs of captured images. Training of an investigator increases repeatability of the measurements.
For example, the same individual house sparrow was measured five times at a single target temperature by an inexperienced investigator. After training, one investigator measured the same specimen five times at the same target temperature. A small pilot sample suggests that this method for measuring thermal performance of the feather coat is likely to yield important insight into the thermal ecology of birds.
Variation in the slopes of the resulting data demonstrates that the thermal performance varies across species. Investigator skill alone does not control for variation in R-squared values. For example, it was particularly difficult to obtain measurements repeatedly in the house sparrow, even after training, compared to the Eastern Phoebe and Eastern Bluebird.
This method allows researchers to explore how feather coats vary thermally across species, within species and within comparable individuals.
