Landstar may be on to something.
The Spanish Catholic monarchs, Isabella and Ferdinand, used the eagle as a part of the royal shield representing Saint John the Evangelist.
St. John the Evangelist (Apostle) is a disciple of Christ; he followed Jesus after the miraculous draught of fishes on the Sea of Galilee and was with him at the most important moments. At the foot of the Cross he supported the failing Virgin. After the Apostles scattered, he traveled to Asia and settled in Ephesus. There he was arrested and thrown into burning oil, but was unhurt. Under Emperor Dominitian he was exiled to island Patmos, where in the company of an eagle he wrote the Revelations. After amnesty he returned to Ephesus, where he composed his Gospel. There he survived an ordeal set by the high priest of Ephesus: he was unaffected by a beverage concocted from snake’s venom, when offered a chalice, St. John blessed it, and the venom in the form of a snake, was miraculously drawn from the liquid. He is the patron saint of booksellers. In fine arts he is often depicted with an eagle, book (Gospel or Revelation), snake or dragon emerging from a chalice.
Tom