Ellicott's Bible Commentary:
“In a bodily shape.—The words are peculiar to St. Luke, and tend to confirm the traditional symbolism which finds in the dove the emblem of the Holy Spirit. They, at least, fall in naturally with this view; but the other construction, that the Holy Spirit descended, after the manner of a dove, first hovering and then resting, in a bodily form (undefined) of some sort, is, at least, not excluded.”
Gill's Exposition of the Entire Bible
“like a dove upon him; either in the form of a dove, or this corporeal form, whatever it was, descended and hovered on him as a dove does:”
And this from a blogger:
“However, I believe the Holy Spirit does not look like a dove any more than Jesus looks like an actual lamb. I believe that when Scripture states it was "like a dove," this expression describes the manner of the descent, rather than the bodily form. So another way to phrase this would be "while He was praying, heaven was opened, and the Holy Spirit descended upon Him like a dove. He was in bodily form."
Actually Matthew records it more like this in his gospel. He states: "After being baptized, Jesus came up immediately from the water; and behold, the heavens were opened, and he saw the Spirit of God descending as a dove and lighting on Him, and behold, a voice out of the heavens said, "This is My beloved Son, in whom I am well-pleased." (Mat 3:16-17)
How does a dove descend? It descends different than other birds. Perhaps we could say it hovers, floats downward, rather than swooping down to kill its prey as a an eagle, or circling overhead like a vulture, or flitting about recklessly like a bat. I believe there was a smooth, steady, deliberate, innocent, pure, and harmless manner in which the Holy Spirit descended on our Lord that can only be compared to the beauty of a white dove descending. Matthew says, "descending as a dove and lighting on Him," indicating that the Holy Spirit came to rest and remain upon Him in the same manner as a bird would light upon Him or land on Him.”
