How do seagulls know when a field is being ploughed?

Like many birds, gulls rely on their Excellent vision in order to find food. Usually, they will survey an area while soaring, and upon spotting a food item, they will swoop down and grab it.

The way gulls actually obtain their food after finding it varies. For the most part, they are scavengers and will pick up whatever's lying around (fish, sea creatures, bread, garbage, or carrion). Many times, they will directly steal food from other birds or animals. Sometimes, however, they will catch their own food by snatching fish from the ocean or snapping up insects.

-An experienced birdwatcher.