Question about bird feeding.?

I live in an area by a lake and this time of year we have ducks, seagulls, and other girls coming back around so I put out bird seed and bread. Her is my question. A small piece of bread was on my garage roof. A seagull spotted it from I don't know how far and he swooped down, grabbed it, and kept going. How was the bird able to tell that quickly it was food?