Ravens, like foxes, are traditional tricksters. Season 5 introduced ravens as a motif starting from the very first shot of 5x01 - Creatures of the Night, when a raven lands on the gate of Eichen House. They appear again during the death of Belasko, flying out of his body like the nogitsune flies once flew out of Stiles's body. In 5x02 - Parasomnia, Tracy vomits up a raven feather along with black ooze and Parrish finds her roof full of dead ravens, just where she'd imagined one trying to get into her skylight.
It is unclear at this time what ravens signify.