Objection 1. It would seem that the dove in which the Holy Ghost appeared was not real. For that seems to be a mere apparition which appears in its semblance. But it is stated (Luke 3:22) that the "Holy Ghost descended in a bodily shape as a dove upon Him." Therefore it was not a real dove, but a semblance of a dove.
Objection 2. Further, just as "Nature does nothing useless, so neither does God" (De Coelo i). Now since this dove came merely "in order to signify something and pass away," as Augustine says (De Trin. ii), a real dove would have been useless: because the semblance of a dove was sufficient for that purpose. Therefore it was not a real dove.
Objection 3. Further, the properties of a thing lead us to a knowledge of that thing. If, therefore, this were a real dove, its properties would have signified the nature of the real animal, and not the effect of the Holy Ghost. Therefore it seems that it was not a real dove.
On the contrary, Augustine says (De Agone Christ. xxii): "Nor do we say this as though we asserted that our Lord Jesus Christ alone had a real body, and that the Holy Ghost appeared to men's eyes in a fallacious manner: but we say that both those bodies were real."
I answer that, As stated above (Question 5, Article 1), it was unbecoming that the Son of God, who is the Truth of the Father, should make use of anything unreal; wherefore He took, not an imaginary, but a real body. And since the Holy Ghost is called the Spirit of Truth, as appears from John 16:13, therefore He too made a real dove in which to appear, though He did not assume it into unity of person. Wherefore, after the words quoted above, Augustine adds: "Just as it behooved the Son of God not to deceive men, so it behooved the Holy Ghost not to deceive. But it was easy for Almighty God, who created all creatures out of nothing, to frame the body of a real dove without the help of other doves, just as it was easy for Him to form a true body in Mary's womb without the seed of a man: since the corporeal creature obeys its Lord's command and will, both in the mother's womb in forming a man, and in the world itself in forming a dove."
Reply to Objection 1. The Holy Ghost is said to have descended in the shape or semblance of a dove, not in the sense that the dove was not real, but in order to show that He did not appear in the form of His substance.
Reply to Objection 2. It was not superfluous to form a real dove, in which the Holy Ghost might appear, because by the very reality of the dove the reality of the Holy Ghost and of His effects is signified.
Reply to Objection 3. The properties of the dove lead us to understand the dove's nature and the effects of the Holy Ghost in the same way. Because from the very fact that the dove has such properties, it results that it signifies the Holy Ghost.
Source: Summa Theologica (New Advent)