@croydon - Mood in poetry is most definitely influenced by the writer though. I mean, putting a raven into your poem might be negative or positive depending on the associations you make. Ravens are intelligent and beautiful but they are also carrion eaters and associated with death.
If your raven is muttering in a guttural voice, I think you'd be hard-put to justify that description isn't intended to create a creepy tone. If your raven is described as gentle, or sitting on someone's shoulder and helping them during the day, then perhaps it's meant to be a more positive image.