Answer:
a sense of hope
Explanation:
In these lines, we see that the author is talking about a thrush, which is a kind of bird. In the poem, we first encounter a bleak and cold environment, which is not at all inviting. However, in this undesirable weather, the speaker is able to see a thrush that is singing. This bird brings the author a sense of hope ("some blessed hope, whereof he knew and I was unaware"), as it is a sign of the spring that is inevitably coming.
