If you think you have to rescue a bird, please make sure it is not a fledgling(learning to fly)!! If you have to catch a bird that you think is injured or sick, please look at the wings and make sure they(the wings) are not shafted.Here is a photo of a crow that was dropped of at my vets office (while they were closed ) that didn't need to be rescued.The wings are shafted (red circle) and this crow was not injured,it was just learning how to fly.
If there is no obvious injury or immediate danger, PLEASE leave them alone and just watch them.