South American Tern

This is a medium-sized marine bird characterized by a body length ranging from 40 to 44 centimeters, a wingspan of approximately 80 to 90 centimeters, and a body mass of 170 to 200 grams. During the breeding season, it displays a striking jet-black cap extending down to the nape, which contrasts sharply with its pure white cheeks, neck, and underparts, the latter often showing a pale greyish wash. The upperparts and wing coverts are a uniform pale grey, and its deeply forked tail features elongated white outer feathers. Its most diagnostic features are its strong red-coral bill and legs. In non-breeding plumage, the forehead becomes mottled white, the bill darkens to a blackish or reddish-brown hue, and the legs lose their bright coloration. Juvenile birds are distinguished by a brownish-grey barred pattern on their backs, a dark bill, and dull yellowish-grey legs.

Geographical distribution: Occurs widely along the coasts of southern South America. Its breeding range extends along the Atlantic coast from Bahia in Brazil southward through the coasts of Uruguay and Argentine Patagonia to the archipelago of Tierra del Fuego and the Falkland Islands. Along the Pacific coast, it nests from central Peru south along the entire coastline of Chile. During the non-breeding season, southern populations undergo significant northward migrations, reaching tropical waters in northern Brazil and southwestern Ecuador, following the cold Humboldt Current to the west and the Malvinas Current to the east.

Habitat: Strongly tied to marine and coastal environments. Its primary habitats include sandy beaches, coastal dunes, rocky cliffs, and offshore islands or islets, which provide crucial protection from terrestrial predators. It rarely ventures inland to freshwater bodies, preferring instead protected bays, estuaries, gulfs, and the surf zone along open shorelines where marine life is abundant. The presence of loose substrate such as sand, gravel, or crushed shells is a critical requirement for establishing nesting colonies.

Feeding: Relies on a predominantly piscivorous diet. It feeds mainly on small pelagic fish species swimming near the surface, such as the anchovy, silverside, sardine, and various juveniles of demersal fish. It supplements its diet with small marine crustaceans, juvenile squid, and other pelagic invertebrates. To capture its prey, it uses a highly specialized foraging technique, performing aerial hovering to locate prey before executing a steep plunge-dive to submerge just beneath the surface, seizing the target with its bill and immediately returning to flight.

Behavior: Known as a highly gregarious and social taxon that performs almost all activities in flocks. It aggregates in large numbers for foraging, roosting, and breeding. It is an exceptionally agile and elegant flier, capable of executing complex aerial maneuvers even in strong coastal winds. Outside the breeding season, it is frequently seen resting on sandbars alongside other terns and gulls. Its vocalizations are loud and harsh, characterized by raspy, high-pitched shrieks used for intra-specific communication, territory defense, and alarm calls against potential predators.

Nesting: Breeding occurs in dense colonies that can vary from a few dozen to several thousand pairs nesting in close proximity. The nest is a simple scrape in the sand, pebbles, or sparse vegetation, sometimes lined with small shell fragments, pebbles, or bits of dry seaweed. The clutch typically consists of 1 to 3 eggs, ranging in color from olive to sandy-beige, heavily blotched with dark brown spots to enhance camouflage. Both parents share incubation duties equally for 21 to 24 days. The semi-precocial chicks hatch covered in down and leave the nest scrape within days to seek shelter in nearby vegetation, achieving full flight capability around 30 days after hatching.

Conservation status: Listed globally as a species of Least Concern (LC) on the IUCN Red List. Although it maintains a large global population and a vast geographic range, its nesting colonies are highly susceptible to anthropogenic disturbances. The primary threats facing this taxon include coastal habitat loss driven by urban development and tourism, illegal egg harvesting, disturbance by recreational beachgoers, predation by invasive alien species such as rats, feral dogs, and cats, and marine pollution from plastics and chemical spills.