The bridled tern (Onychoprion anaethetus) is a seabird of the family Laridae. It is a bird of the tropical oceans. The scientific name is from Ancient Greek. The genus comes from onux meaning "claw" or "nail", and prion, meaning "saw". The specific anaethetus means "senseless, stupid". This is a medium-sized tern, at 30–32 cm in length and with a 77–81 cm wingspan similar to the common tern in size, but more heavily built. The wings and deeply forked tail are long, and it has dark grey upperparts and white underparts. The forehead and eyebrows are white, as is a striking collar on the hindneck. It has black legs and bill. Juvenile bridled terns are scaly grey above and pale below.
The Bridled Tern occurs throughout the tropical and subtropical seas of the world (Raffaele and others 1998). It is a resident species in Puerto Rico (Raffaele and others 1998), regularly seen during breeding season in waters off Mona and Monito, as well as in the Cordillera Natural Reserve and Culebra (Kepler 1978, Oberle 2018), but it is an extremely rare spring and summer visitor in Vieques (Gemmill 2015). However, during the breeding season it can also be seen commonly on the north coast of Puerto Rico between Isabela and Barceloneta (Oberle 2018). This species often occurs far offshore (Raffaele and others 1998) or near islets where it breeds (Oberle 2018, Raffaele and others 1998). The atlas fieldwork yielded a total of 37 records within 24 hexagons or 5 percent of the 479 total hexagons (see map). Of the 24 hexagons where this species was found, breeding met the atlas definition of confirmed in 50 percent (12) of the hexagons and possible in 17 percent (4), while the species was observed in 33 percent (8) of the hexagons but without evidence of breeding (see map).Bridled Tern distribution. The map shows the highest breeding code by hexagon and overlaying the ecological life zones in Puerto Rico. Note: percentages may not total 100 due to rounding. 119Bridled Tern/Charrán Monja
The Bridled Tern nests in small colonies on a coral rubble beach or under the protection of vegetation and rocks on isolated islands or cays from April to July, according to previously published reports (Oberle 2018, Raffaele and others 1998). Atlas results indicate that this species mostly breeds in May, June, and July (see chart). Results show that breeding most commonly occurs in the subtropical dry and subtropical moist forest life zones (69 and 31 percent of the hexagons, respectively) (see table and map).
Although the overall population trend of the Bridled Tern is uncertain, it is listed as a species of least concern by the IUCN (BirdlLife International 2019), and locally it is not listed in any of the threatened categories used by PRDNER and USFWS. In Puerto Rico, the Bridled Tern has a protected habitat in land of 6 percent or 23 km2 of the total area covered by the hexagons where evidence of breeding was found for this species (382 km2).