I would never deny almost anything is possible, but "bird stocking" is about the MOST improbable!!! IF anyone actually observed a Kingfisher dropping a "live" species, the chances of the fish survival is virtually nill as the fish would be highly stressed (and probably suffocated and dried) from a "flight", and almost unquestionably injured from the original capture. It would die in hours from fungus, if not within minutes from the other stressors.
The MOST likely cause (absent human stocking), are simple rain events. Fish fry can swim for miles in under 1/4" of water. Any body of water that does not get runoff is still "connected" to other waters during a rain event. Put together 2 or more frequent events, a puddle here and there during drier periods, and fish fry could migrate some vast distances!
