Terns are seabirds that are related to gulls and skimmers. They are medium-sized and very graceful. It is easy to differentiate them from the gulls by their somewhat smaller size and forked tails. STATUS: The Inca tern is listed as Near Threatened by the International Union for Conservation of Nature (IUCN). After the advent of guano harvesting, the birds’ numbers plummeted due to loss of nesting substrate. Today, there are an estimated 150.