Bells help find the bird if you are close, but they don't do a lot of good with a bird that is out of earshot in a split second. You did see how fast that jalapeno bottle rocket flew, right???
Was a time only recently that transmitters couldn't be fitted to a bird that size because the transmitter weighed too much for the bird to get off the ground!!! These new outfits are downright amazing, some even uplink to satellites every so often to allow people to log their migration across continents!