Sirius had a secret girlfriend, and James knew it. The animagus only ever acted this happy when he was going out with someone, but James couldn't figure out who it was - or why Sirius hadn't told him about this bird. James was, after all, his best friend. He should know.
James had a plan though. A clever, ingenious plan. One that was certain to work.
That night, James secretly applied a permanent sticking charm to Sirius's lips while the animagus was asleep.
The next morning, he got the shock of his life when he saw Remus stuck to them.