Its possible that the fish you stocked were contaminated with mosquitofish also know as Gambusia affinis. Hybrid bluegill are mixed Green sunfish and Bluegill and have around 90% sterile but 10% typically can breed and eventually they revert back to Green sunfish over a course of many years. Birds also may have carried over fish eggs on their feet but typically that's not the case.
My problem with the bird carrying fish egg theory is this:
If the eggs are sticky enough to stick to the birds feet, how do they become unstuck?
Will the eggs dry out during flight?
If the eggs aren't sticky, how do they make it? I can't see a bird flying with an egg pinched between two toenails.....
If there was ANY high water events where there was 1/2" or 1" of water on top of the ground, the fish could have just swam to the pond.
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