Ring Billed Gull
The Ring-billed Gull is a winter visitor in Northern Virginia, where it may be found near ponds, lakes, and landfills. It will come to open areas where it can scavenge for food, and may sometimes be seen on the National Mall in Washington, D.C.! Adult birds are mostly white with a grey back and black-tipped grey wings. There is a black patch or ring near the tip of the yellow bill. At 16-21 inches long, they are smaller than the similar and better-known Herring Gull of the seashore, and may be best identified by their yellow, rather than pink, legs.
Ring-billed Gulls spend the winters inland in the southern USA and near the West, East and Gulf coasts into Mexico and in Cuba. So some come to find shelter and food in Prince William County. It would be inaccurate to call these birds "seagulls" because few of this species ever live by water saltier than the Potomac!