I think Yotasuke is jealous, threatened, and wants to look down on Yaguchi. Yotasuke seems to suffer from social anxiety, and expresses himself too strongly in conversation. He thinks Yaguchi doesn't need to do art since if you have social skills, you can express yourself with people, right? So art is supposed to be Yotasuke's way of expression, Yaguchi is doing it needlessly while reminding Yotasuke of his own "faults" (which he probably sees them as). He's wrong, of course. Yaguchi is socially competent, but he still can't express himself socially. Yotasuke wants to think that his art is more genuine, more real, due to his own social issues. So, when Yaguchi scores high, he comes up with the excuse that the school isn't rating fairly, it's rating how well you're learning to get accepted by Geidai.
Yotasuke probably felt the same way about the bird as Yaguchi, he just didn't want to admit that his appreciation for art wasn't deeper than Yaguchi's, since Yaguchi is supposed to be more shallow due to his friendliness. Or something along those lines.