Forget forking out for an expensive telephoto lens. This is how you do it.
Then get your camera with a 24-105mm lens attached and whilst the bird is still stunned you can get as close as your camera will allow ( about 30cm ) and click away.
After about 10 minutes the bird will recover and fly off.
Hopefully tomorrow it will be a Gannet or a Sardinian Warbler, but today it was a House sparrow.