Jim,
Maybe your birds aren't the same as ours in the South. From my observations, there is no doubt that birds are a vector for fish. Do they instinctively do it? I haven't observed any behavior to support this, but it's possible. Nature always seems to find a way. There are many complex symbiotic relationships in nature. Many of these instinctive behaviors are far more astonishing than simply dropping fish or fertile fish eggs in a nearby body of water.
