...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.....
......
My personal favorite scenarios are, in second place, a King Fisher intentionally brought small fish to the pond to stock it for his future use and .......#1 favorite.....a GBH barfed up live fish into the pond!