311 species of birds have been reported. Almost half of them nest in Yellowstone. As of 1999, twenty-six pairs of nesting bald eagles have been documented. Extremely rare sightings of whooping cranes have been recorded; only three are known to live in the Rocky Mountains, out of 385 known worldwide. Other birds, considered to be species of special concern because of their rarity in Yellowstone, include the common loon, harlequin duck, osprey, peregrine falcon and the trumpeter swan.