Crookshanks darted up first; he had evidently pressed his paw to the knot on the trunk, because Lupin, Pettigrew, and Ron clambered upward without any sound of savaging branches.
Sirius thought it would be -- er -- amusing, to tell Snape all he had to do was prod the knot on the tree trunk with a long stick, and he'd be able to get in after me.
He dashed his head against the knotted trunk; and, lifting up his eyes, howled, not like a man, but like a savage beast getting goaded to death with knives and spears.