birds include piping plovers, great egrets and red-winged blackbirds. Seabirds include brown pelicans and several species of gulls and terns. Wooded habitats shelter ruby-crowned kinglets and white-eyed vireos. During the winter the area shelters northern birds, including brant geese and northern saw-whet owls. As a National Seashore, Assateague Island is required by federal law to protect species listed under the endangered species act. Piping Plovers, listed federally as a threatened species, fall under this category and are therefore protected on the island. During their summer breeding period, closures of the OSV zone and North end of the island often occur in order to prevent visitors from disturbing their nests or harming the newborn chicks. Horses Bands of feral horses, commonly known as Chincoteague ponies or as Assateague horses, freely roam on Assateague Island. Although local legend claims that the horses are descendants of animals that survived a shipwreck on the coast, it is more likely that the horses are descendants of domestic horses confined on the island by local residents in lieu of fencing them in on the mainland, where fences were taxed. The salty low-nutrient diet of saltmarsh grasses has resulted in the horses' short stature and bloated abdomens. Two herds are kept separate by a fence at the Maryland-Virginia border. The horses in Virginia are owned by the Chincoteague Fire Department. These are the famous ponies that are herded every July and then swim to nearby Chincoteague Island as a fundraiser called "Pony Penning." In order to ensure healthy ponies to auction off, the firefighters provide them with food, water, and vet care. The Maryland herd is treated as wildlife and are not rounded up or removed from the island, nor do they receive veterinary care. The National Park Service uses birth control to manage the herd size, injecting a vaccine known as PZP via a dart to control pregnancy and to keep the size of the herd within 80–100 horses. Though many of the horses on the island measure under 14 hands (4 ft 8 inches) in height, which would normally qualify them as ponies, most of their stock comes from horse breeds. Thus, despite their stunted height, which is attributed to their salty diet, they are considered horses and not ponies. Though the horses, especially those in the developed area, may appear tame, they have been known to bite and kick park visitors. Visitors should be