Animals won't eat junk food but, we do?

Now this should tell us something! Just about every day I go out to the pond to feed the ducks bread but, this morning I went out with a dozen donuts that I had left over from after my show. Well, normally when I throw bread in the water the ducks come running across the water from across the pond but, when I through the donuts in the water the ducks didn't come running. Now how in the world did the ducks know that the donuts were crap food from across the pond? They were about 20 feet from where I was throwing the donuts in the water. Donuts are fried and covered in sugar glaze or powered with sugar. hehehehe! Fat and sugary simple carbs just don't go together!!!!!! Oh, any way back to what I was saying... I went back in the house and got a loaf of bread, when back outside and AS SOON AS I throw it into the water the ducks came running across the water so fast that you could see their feet coming out from under them!!! It was the funniest thing!! They were running on top of the water just about!! How did they know that that was good food and the other stuff was sh*it?