I once rescued a fly from a puddle it was drowning in, put him somewhere warm to dry and after a few minutes he took off and flew straight into a spiders web....
You can't odds it sometimes
This didn't just happen to you only, its not that you killed a bird, thats the only story, Spirit killed a bird, no (not that you did kill it). There were at least three beings involved, each bringing their own input to the incident, their own karmas. Truth be told you are not that important in the big scheme of things, none of us are
Your intenions were good, it went wrong, learn from it and let it go.