First question is always is this a small adult bird or baby bird?
Second question is what was the bird doing before you discovered it and what has it been doing since?
In this case, the bird in question was an adult who had flown into the window. Most of the time window strikes cause a bird to be temporarily stunned. We suggest putting them in a covered box, or in a pinch a paper bag with a couple of air holes, and letting them rest someplace quiet for an hour. This protects them from predators and the quiet darkness also helps to reduce their shock. After an hour, take the box outside and open it. If the bird can fly out, fine. If not, it is more seriously injured and should come to a rehabilitator for evaluation. This bird was destined to have a better day and flew away after a short rest.
PS. Don't forget that Tazino's Pizza and Salad Bistro in Menomonee Falls is hosting a 10% event for the Wildlife In Need Center this evening! Download a flyer [here] and 10% of your order will be donated to help us care for our wild neighbors! Tell all your friends and we'll see you there!