In heraldry, the eagle is considered to be the king of birds, as the lion is the king of beasts.
It symbolises perspicacity, courage, strength and immortality, and is also the messenger of the highest gods. With these attributed qualities the eagle became a symbol of power and strength in Ancient Rome. Mythologically, it has been connected by the Greeks with the god Zeus, by the Romans with Jupiter, by the Germanic tribes with Odin, and in Christian art with Saint John the Evangelist.
An early depiction of heraldic art from the Codex Manesse, 1304-1340, an anthology of the works of a total of about 135 minnesingers of the mid 12th to early 14th century. For each poet, a portrait is shown, followed by the text of their works. The entries are ordered approximately by the social status of the poets, starting with the Holy Roman Emperor Henry VI, shown here, with a modern example of his arms.
The heraldic eagle was particularly popular in Germanic countries such as Austria, because of its association with the Holy Roman Empire, whose eagle was doubleheaded, representing the East and West of the old Roman Empire.
Matthew Paris, Corpus Christi College, Cambridge, Parker MS 16 fol. 18, c1250, showing the first depiction of the Reichsadler as double-headed in the arms of emperor Otto IV
Heraldic eagles are most often found displayed, with their wings and legs extended, as in the examples above.
The French Imperial Eagle or Aigle de drapeau was a figure of an eagle on a staff carried into battle as a standard by the Grande Armée of Napoleon I during the Napoleonic Wars, bore the eagle with wings inverted.
The arms of Isabella de Castilla, as Princesa de Asturias, 1468-1474
The badge of the Royal Air Force, showing an eagle volant
Since 20 June 1782, the United States has used its national bird, the bald eagle, on its Great Seal; the choice was intended to recall the Roman Republic and to be uniquely American.
The flag of Mexico, showing an eagle sitting on a cactus while devouring a serpent that signaled to the Aztecs where to found their city, Tenochtitlan
French Civic heraldry abounds with images of the eagle
Here are two examples
The arms of Agen, showing an eagle essorant on a nineteenth century illustration, a French cigarette card and a French stamp of 1964
and another stamp
The arms of Nice