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I had an idea for a unique defense mechanism: An animal could pump air into a tough, impermeable air-sac in the body, until it is highly pressurized. Then, when it is attacked, it can release the air, which not only creates a powerful wind, would also rapidly cool the air due to the expansion. This feature would be found in a species of large raptor that lives in cold climates
Would this attack work in the way I think it will, and is it plausible?
Air - or almost any gas - cannot be pressurized much inside an organic structure, not if you want the structure to be also lightweight - which you do since this is a bird after all. So, compressed air expansion would not do very much.
But the attack in itself might work; you just need a different fluid, one that will subtract a lot more heat. Expansion isn't enough - you need a phase change.
So, your raptor might store energy in a special bladder under not too considerable pressure - say about five bars. At low temperatures this might be doable using dimethyl ether (the bird will also need a rete mirabile to prevent body heat seeping into the bladder). When the bladder is spilled, dimethyl ether will boil and evaporate, rapidly cooling whatever it hits to dangerous levels (dimethyl ether is used in cryotherapy; ten seconds' exposure to a ventilated stream is enough to freeze naked skin up to a depth of three millimeters, which is one of the possible treatments for warts).
There really isn't a metabolic pathway for dimethyl ether (methanol itself is an organic poison), but you could handwave it not too awkwardly with some methanol-dehydrating enzyme operating at near body temperature. The raptor might have started by storing "fuel"; the advantage of a small DME reservoir would have driven evolution towards larger and larger reservoirs, and finally a bone-armored bladder; the ability to vent the bladder in case the temperature gets too high would also have evolved; and finally, venting the bladder at will would have turned the whole setup into a weapon.
