Witchipedia

Birds are messengers from Otherworlds in many cultures and so are often seen as carriers of omen. If this incident was a sign, then you will know it. The bird will remind you of someone you know (likely someone in the spirit world or close to it) or it will remind you of an idea or concept. If you don't immediately get that gut feeling, then it's probably not a sign. Omens also usually occur in clusters, just to make sure you get the message. Dreams usually accompany omens as well, so keep your dream journal by your bed so you can write them down quick before you forget!

Birds flying into windows is fairly common. People who have a problem with this issue can put window clings on the windows, to make it really obvious something is there. You can switch them up seasonally, snowflakes in the winter, flowers in the spring and summer, autumn leaves in the fall. That's assuming putting up curtains or blinds won't work for your situation.