Hedwig is Harry's owl. She carries Harry's messages to Sirius Black for him. But because Sirius is in hiding and it would look weird if a snowy owl kept going south (where such owls stand out), Harry can't use her to deliver his mail all the time. She resents this, and nips his fingers when he tries to give his letter to Sirius to another owl. Harry shouts: "First Ron, then you [...] This isn't my fault" (18.36). Still, by and large, Hedwig is very dependable and loyal.