In addition to the Armor spell, you might think about a path like the Shield spell, given that most attacks against the bird will, I assume, be ranged. The +1 Save bonus will also help (since any AoE spell will fry a normal bird).

The bird's AC comes from its speed and maneuverability, so physical armor will likely offset the AC as much as help it.

Phantom Armor (from the 1e Unearthed Arcana) might be another option. It also includes the Save bonus, and damage reduction, both critical to a familiar-like companion at higher levels.