kntombat
IanStewart, I would love to hear your take on this question.
I'm not sure I have anything interesting to say about it

We say "x and y are equal", and there's no reason to say "x and y are equal to each other", so that alone makes C or E better than the other choices. But the other choices also suggest a second meaning that isn't intended: "the two birds appear to each other that they are equal in size". The sentence is presumably describing how the birds look to people, and not how the birds look to each other.
I don't see any major issues with either C or E, except for that semicolon in C, which is clearly wrong (though easy to miss on a quick read). I don't think I've seen the actual GMAT ever take a perfectly good answer choice in SC, and turn it into a wrong answer just by replacing a comma with a semicolon -- that seems more a test of eyesight than of anything else. I think there's one superfluous comma in answer E, but it's not really wrong, just something I'd delete if I was editing it.