Jamie barked because he was very pleased and happy to show off. That was just who he was - a show-off. It had gotten him in more trouble than he could really remember. So many detentions, polishing trophies and what not. But honestly, it was best not to try and figure out the how-to-fors of being an animagus. It just got confusing.
He looked up from chips at the question, one ear perked up more than the other. Very carefully, he shook his head back and forth so the 'no' would be clear. Because he was not actually a dog, after all. Jamie bounced off the couch (there was no other way to describe the way a Border Collie moved) and pranced around with high steps, clearly still showing off how proud he was of his furry form.