Why northern mockingbirds (Mimus polyglottos) face the sun: an evaluation of Thermoregulation and countershading explanations using orientation during overcast conditions

When the sun is near the horizon, Northern Mockingbirds (Mimus polyglottos) show a strong tendency to orient their lighter-colored ventral surfaces toward the sun. It is unclear whether the function of this orientation is visual conspicuousness, thermoregulation, or regulation of exposure to ultraviolet radiation. However, all three of these potential functions would not apply when sunlight is blocked by dense cloud cover. Thus, mockingbird orientation during overcast conditions should be random. To test this prediction, 50 mockingbirds were found during overcast but otherwise calm conditions and the compass orientations of their ventra were recorded. Compass orientations were strongly directional and associated with the sun\u27s azimuth. Overall, mockingbirds appeared to direct their ventra toward the sun under both cloudy and clear skies, but the tendency is much weaker under cloudy skies. Polarization in exposed patches of skylight could be the relevant cues for orientation during overcast conditions. Mockingbirds may have been reinforced for orienting their ventra with respect to the sun and sky polarization. This reinforced orientation may be generalizing to overcast conditions and other contexts in which some orientation cues are present. The benefits derived from this behavior remain unclear