Found a discussion about bird stocking of ponds
here. I have personally watched both means of fish transportation by birds.
The heron I scared away from our pond puked in the pond before he flew away. I have also heard of them puking in the pond to chum the minnows over. I have not seen this but have scared one to the point it puked. I expect if any minnows from another pond were in their belly but not dead they swam away in our pond.
The belted king fisher was moving minnows into a small puddle one day. I watched for some time... thought it was odd. The next day I watched her with her babys in this small puddle leaning to fish. King fishers are called the king because not once did she leave our pond with out a minnow.
Cheers Don.