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The Spectacled Tern (Onychoprion lunatus), also known as the Grey-backed Tern, is a seabird of the tern family Sternidae. This species is characterized by its medium size, typically measuring about 33 to 38 centimeters (13 to 15 inches) in length, with a wingspan of roughly 76 to 81 centimeters (30 to 32 inches). It has a slender, streamlined body adapted for an aerial lifestyle, with long, pointed wings and a deeply forked tail that provides excellent maneuverability during flight.

The plumage of the Spectacled Tern is predominantly grey and white, which is not only aesthetically pleasing but also serves as camouflage against predators and aids in hunting for fish. The upperparts are a pale grey, which contrasts with the white underparts. The bird's common name is derived from the distinctive black markings around its eyes, which resemble spectacles. These markings include a black cap that extends over the eyes, encircling them and creating a striking facial pattern. The black cap tapers off towards the nape of the neck.

During the breeding season, the adults exhibit a more pronounced black cap, which can appear glossier, while the rest of the year, the black markings may be less vivid. The bill of the Spectacled Tern is short and sharp, allowing it to snatch fish from the water with precision. Its bill color can vary from dark to a yellowish hue, depending on the individual and the time of year.

Spectacled Terns are social birds, often found in colonies during the breeding season. They choose to nest on isolated islands and atolls, where they lay their eggs directly on the ground or in shallow scrapes lined with vegetation. Their nests are located in close proximity to each other, providing a collective defense against potential predators.

The diet of the Spectacled Tern mainly consists of small fish and marine invertebrates, which they catch by diving into the water from flight. They are adept foragers, often following schools of fish and taking advantage of feeding opportunities presented by larger marine predators that push prey to the surface.

These terns are migratory, with populations moving across vast stretches of ocean between breeding and non-breeding seasons. Their migration routes and wintering grounds can span across the tropical and subtropical regions of the Pacific Ocean.

Conservation status of the Spectacled Tern varies by location, but habitat destruction, human disturbance, and climate change pose threats to their breeding sites and overall population stability. As a result, conservation efforts are important to ensure the protection of their nesting areas and to monitor the health of their populations.

In summary, the Spectacled Tern is a graceful and striking seabird, whose life is intricately connected to the marine environment. Its distinctive markings, agile flight, and social nature make it an interesting subject for birdwatchers and an important species for marine ecosystem health.