It is very attractive to think that this form of eagle was chosen, as has been suggested, to express the fact that Constantinople was now lord over both halves, East and West, into which Diocletian had divided the original empire of Rome. Whether this idea was behind the choice I do not know, but at any rate the two-faced eagle became latterly the acknowledged ensign of imperial Byzantium, and as such was introduced into European royal heraldry, whether or not by means of the returning Crusaders, as commonly stated, remains obscure.
In the 15th century what was left of the Holy Roman Empire became the heritage of the Austrian house of Hapsburg which had succeeded the German Hohenstauffens; and to Sigismund, head of the house in that century, is ascribed the design in the Austrian arms of the two-headed eagle, looking right and left, as if to signify boastfully that he ruled both East and West. These were relative and indefinite domains, but as he had, by his crowning at Rome, received at least nominal sovereignty over the fragmentary remains in Greece of the ancient Eastern Empire, he was perhaps justified in adopting the Byzantine ensign as “captured colors”; but a rival was soon to present a stronger claim to these fragments and their badge.
In this same period, that is in the middle of the 15th century, Ivan the Great of Russia was striving with high purpose and despotic strength to bring back under one sway the divided house of Muscovy, together with whatever else he could obtain. To further this purpose he married, in 1472, Sophia Paleologos, niece of the last Byzantine emperor, getting with her Greece and hence a barren title to the throne of the Eastern empire—a barren title because its former domain was now over-run by the Turks, but very important in the fact that it included the headship of the Greek, or Orthodox, Church. From this time Russia as well as Austria has borne a two-faced eagle on its escutcheon; and, although both birds are from the same political nest, the feeling between them has been far from brotherly.
It may be remarked here, parenthetically, that in Egypt the cult of the kingly eagle never flourished, for the griffon vulture, “far-sighted, ubiquitous, importunate,” became the grim emblem of royal power; and a smaller vulture (_Neophron percnopterus_) is called Pharaoh’s chicken to this day by the fellaheen. By “eagle” in Semitic (Biblical) legends is usually meant the lammergeier.