Campaign for Shades of Darkness: The Fall.

Hmm... I don't know what's going on here. I tested it three times, and everything was fine. Try reloading the battle with the bird (there's only one, by the way), if you have a save. If that doesn't work, you'll get the Boots of Speed as a reward, so this quest isn't critical.
Also, try installing patch 3.2 (if you're playing Heroes 3: The Shadow of Death); it's available on this website. Perhaps, you'll be able to avoid the quest failing in the future.