I read that the San Diego Zoo acquired some animal that was so rare (perhaps none in captivity before that) that no one knew their temperature requirements.
So they gave the pool a "temperature gradient" where one part was warmer, and another part cooler, than the rest.
They watched where the animal (a rhino?) preferred to hang out and adjusted the temperature to match what it liked.
If you gave your enclosure one heated corner, or a heated box with a small entryway, the birds would use it whenever they felt cold.
