Why dom seagulls have a large wingspan?

Seagulls have large wingspans for the same reasons that gliders do. Long and slender (high aspect ratio) wings produce less drag for a given amount of lift (at least at slow speeds) than short, stubby ones. Seagulls use these efficient wings to allow them to stay aloft with ease while taking advantage of updrafts caused by winds deflected by cliffs, buildings, even waves. In windy conditions, seagulls can use waves to stay aloft by engaging in what is known as "dynamic soaring" - flying into and out of regions of higher and lower wind speeds.