About Thalasseus bernsteini (Schlegel, 1863)
Thalasseus bernsteini is a medium-large tern that measures 38–43 cm in length, and it closely resembles the Sandwich tern (T. sandvicensis), Cabot's tern (T. acuflavidus), and lesser crested tern (T. bengalensis) in appearance. It is most similar to the Sandwich tern, but differs in its bill pattern: the bill is yellow with a black tip, which is the reverse of the Sandwich tern's bill pattern. Its bill is also stouter, matching the bill shape of Cabot's tern. Like the three similar species, it has a full black crown during the breeding season, a pale silvery-grey back and wings, and a white tail. When compared to the lesser crested tern, which shares the same wintering distribution range, this species can be distinguished by its white rump, paler grey mantle, stouter bill with a black tip that also has a visible white point when viewed up close. The larger greater crested tern, despite being the closest relative of Thalasseus bernsteini, differs more clearly: it has an all-yellow bill, a white forehead in the breeding season, much darker grey mantle and rump, and a larger overall body size. This species is classified as critically endangered, and was previously considered extinct. In 2000, four breeding pairs were rediscovered nesting within a greater crested tern colony on an islet in the Matsu Islands, a territory governed by Taiwan located just off the coast of Fujian Province, China. The species winters as far south as the Philippines. Historically, it had a wider distribution along China's east coast, extending north to Shandong Province. Its population decline is thought to be caused by historical hunting and egg collection for food. The existing Matsu Islands colony may have been protected due to the islands' disputed status: they are administered by the Taiwanese government but claimed by mainland China, and the area's military sensitivity has restricted public access. The islet where the colony is located has now been designated a wildlife sanctuary. It is possible that other small colonies may yet be discovered off the coasts of China and Taiwan; migrating individuals have been observed near the mouth of the Pachang River in southern Taiwan. The total global population is estimated to be fewer than 50 birds. In 2007, it was projected that the Chinese crested tern would go extinct within five years without protective action from authorities. BirdLife International reported that a survey conducted by Chinese experts found the population of this tern had fallen to 50 birds, half the population size recorded in 2004. A Chinese survey team led by Chen Shuihua stated that the bird was "on the verge of extinction". Currently, the species is threatened by illegal egg collection, typhoons, and disturbance of nesting colonies by fishermen. It also faces a threat from hybridisation with the greater crested tern. In 2016, Chinese crested terns were recorded breeding in South Korea for the first time. Establishing a new colony this far from the species' known breeding range is considered a positive development for the species.
