Assalaamu alaykum. I hit a bird with my car about 20 minutes ago on a road that sees a good deal of traffic. It is probably impossible for me to do anything about that now, but I would like to know what I was supposed to do. When I hit the bird, I kept going because I had to get to work and I assumed that even if the bird was still alive, after that blow it would most likely die soon anyway. It is probably impossible for me to do anything about this now, but I would still like to know what I was supposed to do.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

You bear no sin for what you did unintentionally, even if you had verified the death of that bird. The Permanent Committee for Islamic Research and Fataawa was asked, "A man unintentionally hit a cat with his car, what should he do?"

The answer reads, "There is no harm on the one who unintentionally killed the cat, for unintentional killing in such a case and similar ones is overlooked."

It seems from your question that the bird probably died and that if it was still alive, it would eventually die because of the blow. In this case, it would have been recommended for you to slaughter it if it was still alive in order to spare it the suffering and relieve it from its pain by giving it a quick death. Some scholars held that it is recommended to slaughter the injured or sick animal if there is no hope of recovery in order to relieve its pain, even if its meat is unlawful for consumption. Sharh Az-Zarqaani ‘ala Mukhtasar Khaleel Al-Maaliki reads: 

"Slaughtering the animal whose meat is unlawful for consumption according to the Sharia: It is allowable, even recommended, to slaughter the animal if there is no hope of recovery of a sickness or blindness or in a condition of the same meaning, as is the case when it is in a deserted place where no food would be available for it and there is no hope that someone may take it and feed it. The same applies to a camel that has become injured during a journey and cannot travel and one cannot avail himself of its meat; he may slaughter it unless he fears for the safety of whoever would eat its flesh, as underlined in Al-Waadhihah. Hence, he should slaughter such a camel unless he fears for whomever might eat its flesh among the passers-by after slaughtering it, giving priority to preserving the life of the sane being (human beings) over preventing the torture of the animal by killing it, despite its bad condition..."

In any case, you bear no sin for accidently causing the death of this bird, even if you left it and did not slaughter it according to the Sharia.

Allah knows best.