To reproduce:

1. Put up wind walk while in human form and without flight spell so you get both affects.

2. Shape into your avian form.

3. Go into a room that causes windwalk to go away. In my case, I landed into a city square from high altitude.

The result is that your bird form no longer has flight. Mobs can trip you and you can't move into water rooms without putting up windwalk again or enlivening flight.

Your bird form won't lose flight anymore.