Michael - good observation. Come to think of it, whenever I see a bird's breath
steaming it is likewise when the bird is vocalizing. (Magpies being good study
subjects around my neighborhood).

This is speculation, but maybe it has to do with the 4-step process their breath
goes through, which I understand is:

1) Inhale: air enters anterior air sac system
2) Exhale: same bolus of air enters lungs
3) Inhale: same bolus enters posterior air sac system
4) Exhale: same bolus exits bird

The posterior air sac(s?) may act like heat exchangers.

When the bird vocalizes, it apparently interrupts the normal flow pattern.

Chris Maack
Bird Treatment & Learning Center
Anchorage, AK
73201.3124 at compuserve.com