Actually I've been asked this quite a few times while showing raptors around the airport, "Why is it's mouth open?" Birds do not have that awesome cooling mechanism that you and I have -- they don't sweat. Much like your dog at home they pant which allows saliva in their mouth and on their tongues to evaporate. It's not quite as effective as our sweating due to the decreased surface area available for evaporation but it's better than nothing. Birds will of course take advantage of any available water to bath in and cool down. They may also do so by soaring to where air is cooler, after all referring to the dry adiabatic lapse rate from back in ground school days we know that air with less than 100% relative humidity (and barring inversion) lapses at a rate of 5.4°F/1,000' or -14.7777778°C/ 304.8m -- so yeah it's [typically] cooler the higher you go. Birds may also ruffle their feathers allowing the both a breeze to get under their feathers and the built up heat from their body to escape. There's even a hypothesis that the size of a bird's beak may be design of thermoregulation.
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