Why ysk: most birds that don't die instantly on impact die in the next few days from their injuries, even if they manage to fly away. Birds on sidewalks or near buildings that seem dazed, don't fly away when you approach, or look unusual (injured or not the type of bird you typically see on city sidewalks like pigeons or sparrows) have hit a building, often during migration, and will die without appropriate professional care.

Instead of waiting to see if the bird is ok, immediately place the bird in a closed paper bag or box with air holes, handling it as little as possible. Do not give the bird any food or water, but keep it in a warm, dark, quiet place until you can bring it to a licensed wildlife rehabber.

edit: i made this post because during fall migration season, i often receive phonecalls and messages at work asking what to do about dazed birds. you can see plenty of posts where people stumble on window strike birds on r/whatisthisbird. this is literally common knowledge to anyone who works with or around wildlife. I've added sources below for anyone who doubts this as the recommended course of action. also yes, obviously you should wash your hands.

Tufts Vet Clinic Bird Strikes Audubon: You found a bird that crashed into a window. Now what?