Wild animals— particularly rabbits, fawns, and birds— are prone to a condition called capture myopathy. This is a complex disorder that results from the stress of being chased, captured, or even simply held by a human being. Although there are usually no signs of disease or injury, these animals simply die.
Please: never capture or handle an animal unless instructed to do so by a wildlife rehabilitator. Otherwise, you run the very real risk of killing an animal that you intended to save.