Bridled Tern

Bridled Tern
Photo by seabamirum. Licensed by CC BY 2.0.

Bridled terns are medium-sized tropical terns, with long, slender wings and a deeply forked tail. They reach a little over a foot in length with a 2.5-foot wingspan. They are sooty brown or grey above and white below, with black on the crown and a white forehead and eyebrow. The bill is about as long as the head and is black in color, as are the legs and feet. Outside of the breeding season, the bridled tern is slightly paler, with its black crown streaked white and brown, and the dark feathers of the back having white tips, giving it a peppered appearance. Immature birds are broadly similar to the adults, but the back is grayer with pale feather edges, giving it a scaly pattern. Juveniles also have an all-dark face and breast with white only on the belly. In appearance, bridled terns are most similar to sooty terns, but they have very different strategies on how to survive in the nutrient poor waters of the tropics. The flight pattern is also more light and buoyant, and the bridled is a lighter and more delicate build. In the wild, bridled terns have a lifespan of around 12 to 18 years. The scientific name, Onychoprion anaethetus, is from Ancient Greek. The genus comes from onux meaning "claw" or "nail,” and prion, meaning "saw.” The species, anaethetus, means "senseless” or “stupid,” apparently a reference to the ease with which hungry sailors captured this relatively docile seabird.

Most populations are migratory and disperse at the end of the breeding season to overwinter at sea. Away from the breeding grounds, the species is entirely pelagic, perching occasionally on floating objects from sargassum to fishing boats. They are fairly common summer residents to pelagic Texas waters – found in warm tropical waters worldwide, and rarely seen on land during non-breeding seasons, except when blown off course. Bridled terns are generally silent at sea but can be noisy in breeding colonies and roosts. They have a variety of short harsh calls and also a yapping call reminiscent of a small dog.

As seabirds, small schooling fish are their main source of food, with some squid and crustaceans. They generally find their meals where seaweed and flotsam gather, such as current convergences. Sometimes, they concentrate where schools of predatory fish are chasing smaller fish to the surface. They hunt mostly by flying low, hovering, and dipping down to take items from the surface of water; they rarely plunge-dive into the water.

Bridled terns usually nest in noisy colonies, often associated with other tern species. They are seasonally monogamous. Courtship involves high flight by groups or pairs. Males fly slowly and low over the colony, carrying a stick or fish, pursued by other birds. On the ground, the pair bow, strut, and turn in circles, and the male offers fish to the female. Nest sites are on rocky islands or exposed reefs, with a periphery of rubble, bushes, or other shelter. The nest is a scrape or depression in shingle or sand with little or no lining, either freshly excavated or re-purposed from a previous season, concealed in a variety of locations, including natural cavities amongst rocks or coral rubble, vegetation, crevices or caves, under cliff ledges, etc. Though not strictly a colonial species, pairs usually congregate in suitable habitats with neighboring nests spaced according to nest site availability (usually 3-15 feet apart). The female lays only a single egg. Both parents incubate for about a month. Chicks leave the nest after a few days to hide in nearby cover. Both parents also feed the young, regurgitating small fish. The young fledge in about 2 months and are independent about a month later.

Bridled terns have been known to abandon breeding colonies when subject to severe human disturbance, although they can become habituated to human presence in sites exposed to continuous visitation, especially where human movements are predictable and reliable, and groups sizes are kept consistent. Additional measures to reduce human disturbance of nesting colonies include the erection of barriers and signs, provision of walkways, and supervision and education of visitors. Eggs are harvested for subsistence in the Bahamas and the West Indies; both eggs and chicks are harvested on some islands in the Pacific by local residents and coastal shipping crews. Neither of these are considered to represent a significant threat to the population. Though the population trend is unknown, it’s range is very large, and it is not believed to be decreasing sufficiently rapidly for concern. The global population is estimated to number 610,000-1,500,000 individuals, roughly equating to 400,000-1,000,000 mature individuals. For these reasons, the IUCN Red List lists the bridled tern as a species of Least Concern.