Eagle/1.03

Eagles are basic fliers. The Eagle's specialty is that it can build from both building lists and, by being a flier, you wont even have to worry about the Eagle trapping itself after building too closely to another object. They're pretty bulky and have a high combat stat for a basic flier, so they can certainly hold their own against others and are likely to come at on top.

Barbarians
The Eagle's specialty is that it can build from both building lists and, by being a flier, you wont even have to worry about the Eagle trapping itself after building too closely to another object. Their build skill is also higher than the Thrall's, and with a faster movement and flight, the Eagle is vastly superior to the Thrall when it comes to direct building, making it the builder of choice when available. Their combat capabilities however, aren't that great and even a large flock of them is nothing more than just a flying distraction. The fliers that causes problems for the Barbarian infantry also cause similar problems to the Eagles, so they can't even help in those situations.

Benifits from Training, Eagle Eye, Magical Tattoos, and Bloodlust researches.

Dwarves
The Eagle's specialty is that it can build from both building lists and, by being a flier, you won't even have to worry about the Eagle trapping itself after building too closely to another object. They're pretty bulky and have a high combat stat for a basic flier, so they can certainly hold their own against others and are likely to come at on top. But due to the Dwarves having access to such fast builders, the Eagle should only be used for long distance building, where its speed and ability to fly would allow it to reach such destinations minutes ahead of a Dwarven Smith.

Benifits from Armorer and Mithril researches.

Empire
The Eagle's specialty is that it can build from both building lists and, by being a flier, you wont even have to worry about the Eagle trapping itself after building too closely to another object, as well as quickly getting to the destination of the construction site by ignoring any terrain in the way (barring cavern walls). As their build skill is only a single point lower than the Peasant's, they are, simply put, overall better than Peasants in regards to building, but only during the late game where you have need to build long distances away from locations.

Eagles are also somewhat capable of combat and can quite easily fend off other basic fliers and even Imps. The triple damage vs larger flying units also helps out, but as much as the Empire could do with additional aerial support, Eagles are still pretty basic and their abilities can only go so far. As such, Eagles should only be used as support along side Archers to help fight off large swarms of enemy fliers or when playing against Daemons, and not used as a dedicated form of attack.

Benefits from Armorer research.

Knights
The Eagle is accessible at the level 1 flier building. It may cost quite a lot to produce for a basic flier, but it can build buildings and is the toughest basic flying unit in the game by a considerable margin. Still, it is usually not as powerful as Firebats.

Benefits from Crusade, Armorer and Order of the Rose researches.