If he could manipulate Fenwick like that, who's to say he couldn't take his time and work other griffons over to his side as well?  That wren in the story, none of the other birds saw him sneak up behind the eagle's back after all, and after the eagle had done all the hard work, the Wren stole the crown. If any did, none of them ratted him out. He'd have to be like the wren, either fool everybody or make it in their best interests not to get in between him and what he wanted.