A truck stops at a weigh station at the entrance to a bridge and is shown to weigh exactly 2000 lbs. The operator of the weigh station notes that the bridge can hold exactly 2000 lbs, but would crumble if it were subjected to even a fraction of an ounce more. But he says that no other vehicles ever cross over the bridge and so it's fine if the truck crosses. The truck continues on.

Once the truck is halfway across the 10-mile long bridge, a sparrow flies over to the truck and lands on the hood. The truck driver sees the bird lands and his heart drops as he realizes the bridge is about to collapse, but even after the bird lands, the bridge doesn't crumble.

How was the bridge able to hold up despite the extra weight from the bird?