You can look this up on 365chess.com, which is what I did. Below are two of the most popular lines they gave, though only one game had 7. a4. White won 100% of the games after both of these end positions, so this Bird's Defense evidently isn't very good for Black.
You're welcome. Such NxN captures occur in other openings, too. One YouTube video narrator called that Nxd4 move "wimpy" in the Four Knights Game, but another YouTube video narrator recommended similar moves if a player wants to reduce the opponent's chance of winning through tactics or deep book knowledge. Such captures simplify things immediately.
The modern trend on the Bird is 4.Nxd4 exd4 5.Bc4, which stops the thematic 5...Bc5 due to the cheap trick 6.Bxf7+.
http://www.chess.com/echess/game?id=121282332