Fish egg hopped a ride on a bird.You may end up with some unintended fish. After a recent polar vortex froze my pond solid down to the ground, I lost all of my koi and most of my interest in maintaining it. Its not big and I only go back there to soak my trees when repotting. Well today, as I was doing that very thing, I caught the flicker of a bluegill darting for the bottom. Now my pond is tiny, no larger than 10 feet at its longest point and its completely overgrown with iris and Lily. Also, I live on a city lot. Where the hell did the bluegill come from?
I once owned a mini farm that had a mud pond. Fed via rain runoff. There were fish in it, but it had never been stocked. The birds did it.