The least tern (Sternula antillarum) is a species of tern that breeds in North America and locally in northern South America. It is closely related to, and was formerly often considered conspecific with, the little tern of the Old World. Other close relatives include the yellow-billed tern and Peruvian tern, both from South America.
The Least Tern is widespread and local in North America (Raffaele and others 1998), and winters off Central and South America (Oberle 2018). In the West Indies, it is a common breeding resident in The Bahamas, Greater Antilles, Cayman Islands, St. Martin, Antigua, and Barbuda mostly from May to August (Raffaele and others 1998). However, it is an uncommon breeder in the Virgin Islands and some of the other northern Lesser Antilles south to St. Christopher, while it ranges from uncommon to rare among the more southern Lesser Antilles (Raffaele and others 1998). It is described as fairly common in the coastal areas of Puerto Rico, especially on the southwestern region where colonies of this species nest near the Cabo Rojo lighthouse (Biaggi 1997). On Vieques, it is a fairly common breeding visitor in summer, rare in fall, extremely rare in winter, and uncommon in spring (Gemmill 2015). It usually inhabits shallow coastal waters (Oberle 2018), harbors, and lagoons (Raffaele and others 1998), and more recently has been nesting on the roofs of shopping centers. The atlas fieldwork yielded a total of 82 records within 46 hexagons or 10 percent of the 479 total hexagons (see map). Of the 46 hexagons where this species was found, breeding met the atlas definition of confirmed in 33 percent (15) of the hexagons, probable in 7 percent (3), and possible in 9 percent (4), while the species was observed in 52 percent (24) of the hexagons but without evidence of breeding (see map).Least 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. 121Least Tern/Charrancito
Previously published reports indicate that the Least Tern breeds from April to July (Raffaele and others 1998). The nest consists of a scrape that can be located in a wide variety of habitats ranging from industrial sites (Raffaele and others 1998) and gravel-roofed buildings to undisturbed beaches (Oberle 2018), and Least Terns either nest as single pairs or in colonies (Raffaele and others 1998). Atlas results show that this species breeds from April to August with the most breeding activity from May to July (see chart). Overall, the breeding activity peaks during May and June, and it mostly takes place within the subtropical dry forest life zone (see chart). Atlas results show that this species breeds mostly in southern coastal areas within the subtropical dry forest life zone (91 percent of the hexagons) and less commonly in the subtropical moist forest life zone (9 percent of the hexagons) (see table and map).
The current overall population trend of the Least Tern is described as decreasing, although some populations are increasing and others have unknown trends (Delany and Scott 2006). The Least Tern is currently listed as a species of least concern by the IUCN (BirdLife International 2018), and locally it is not listed in any of the threatened categories of PRDNER and USFWS. As a ground nester, the nests are subject to nest predation by mongooses (Herpestes auropunctatus), cats (Felis spp.), dogs (Canis spp.), and humans, which may cause breeding birds to abandon nests. In Puerto Rico, the Least Tern has a protected habitat in land of 18 percent or 94 km2 of the total area covered by the hexagons where evidence of breeding was found for this species (527 km2).