South American Tern

Sterna hirundinacea

Order: Charadriiformes / Family: Laridae

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white black gray medium waterbird seabird coastal migratory colonial common widespread

About the South American Tern

The South American Tern, Sterna hirundinacea, is a prevalent seabird found along the coasts of South America, particularly in Argentina, Chile, and Brazil. This species thrives in marine environments, often seen in large flocks along sandy beaches, rocky shores, and occasionally inland waterways. It plays an essential role in the ecosystem by controlling fish populations and serving as a bioindicator for marine health.

The South American Tern is well-adapted to its coastal habitat, using its agility and speed to forage and evade predators. During the breeding season, these terns congregate in large colonies, enhancing their survival odds through shared vigilance and predator deterrence. Outside the breeding season, they disperse widely, with some individuals traveling northwards up the Atlantic coast, showing a flexible response to environmental conditions.

This tern's ecological role extends beyond predation. By consuming small fish and invertebrates, it helps maintain the balance of aquatic ecosystems. Additionally, its nesting habits contribute to the nutrient cycle on coastal islands, where guano enriches the soil, supporting plant life and other organisms. The South American Tern, through its presence and activities, underscores the complex interdependencies within its ecosystem.

The South American Tern belongs to the family Laridae, within the order Charadriiformes. These birds typically live average 10 years, with a max recorded up to 20 years..

How to Identify a South American Tern

The South American Tern is a medium-sized bird with a streamlined body and a length reaching about 35-40 cm. Its most distinctive field marks include a black cap extending from its bill to its nape, contrasting sharply with its white forehead and underparts. Adults display a pale gray back and wings, with a deeply forked tail that is white, allowing easy identification in flight.

Juvenile terns resemble adults but have mottled brown and gray plumage on their backs and wings, which gradually transitions to the adult's sleek gray as they mature. Male and female terns appear similar, though subtle differences may exist in size, with males typically slightly larger. Observers may confuse this species with the Common Tern, but the South American Tern has a longer tail and differs in its migratory patterns.

In flight, the South American Tern exhibits a buoyant, graceful flight pattern, with rapid wingbeats and glides. When perched, they maintain an upright posture, often seen resting on coastal rocks or driftwood. Their slender bill and long legs are adapted for their foraging lifestyle, making them efficient predators of small fish and invertebrates.

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South American Tern Habitat and Range

Preferred Habitat

The South American Tern primarily inhabits coastal regions, favoring sandy beaches, rocky shores, and estuaries. In the breeding season, they prefer remote islands and islets with minimal human disturbance, which offer safety from terrestrial predators. Outside of the breeding season, they can adapt to various coastal environments, including river mouths and tidal flats.

These terns typically avoid urban areas, though they may be sporadically observed near ports or harbors if food resources are abundant. They are often found at sea, foraging far from shore, especially during non-breeding months. They occupy a range of elevations from sea level to low coastal cliffs, demonstrating a preference for open, unobstructed views of the ocean.

Seasonal movements influence their habitat choices. During the austral winter, when northern populations migrate towards warmer regions, they may temporarily inhabit more northern coastlines, showcasing their adaptability to different marine conditions.

Geographic Range

The South American Tern breeds along the southern coasts of South America, with key nesting sites in Argentina, Chile, and the Falkland Islands. During the austral summer, breeding colonies can be dense in these regions, particularly in Patagonia and Tierra del Fuego.

In the non-breeding season, their range broadens considerably. Some populations migrate northwards along the Atlantic coast, reaching as far as the southeastern coast of Brazil. This migration highlights their adaptability and reliance on diverse marine environments.

While not as extensively migratory as some other tern species, the South American Tern's movement patterns are significant for understanding its ecological niche. Population density hotspots include the nutrient-rich waters off the coast of Argentina during the breeding season, where they take advantage of abundant food resources.

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What Does the South American Tern Eat?

The South American Tern primarily feeds on small fish and invertebrates, exhibiting opportunistic foraging behaviors. Fish species such as anchovies and sardines constitute a significant part of their diet, especially during the breeding season when energy demands are high.

During other times of the year, their diet may diversify to include crustaceans and cephalopods, depending on availability. They hunt using a method known as plunge-diving, where they spot prey from above and dive into the water to capture it. This technique requires precision and speed, making these terns highly skilled hunters.

Prey size typically falls within the 3-5 cm range, aligning with their medium bill and efficient handling capabilities. Feeding occurs throughout the day, influenced by tidal cycles and prey activity, with peak foraging often observed during early morning and late afternoon.

South American Tern Behavior and Social Structure

South American Terns exhibit complex social structures, often forming large breeding colonies that provide safety in numbers. These colonies are highly vocal, with individuals communicating through a variety of calls to establish territory and coordinate activities. During breeding, they display courtship behaviors such as fish offering and aerial displays, which strengthen pair bonds.

Outside the breeding season, they may gather in smaller flocks, showing less territorial aggression. These flocks can frequently be seen foraging together, a strategy that increases foraging success and predator awareness. The South American Tern is diurnal, with most activities occurring during daylight hours, though they may continue foraging during bright moonlit nights.

In response to predators, these terns exhibit aggressive mobbing behavior, collectively harassing intruders to protect their nests and young. This defensive strategy is crucial during the breeding season when vulnerability is highest.

South American Tern Nesting and Reproduction

South American Terns prefer to nest in colonies on isolated islands or remote coastlines, where they construct simple scrape nests in sand or gravel. Nests are often lined with small stones, shells, and plant material to offer some insulation and camouflage.

Clutch size usually consists of two to three eggs, which both parents incubate for about 22-24 days. The incubation period requires constant vigilance from the parents to protect against predators and environmental conditions.

Once hatched, chicks fledge in approximately 25-30 days, during which time both parents provide food and protection. Parental roles are well-defined, with both engaging in feeding and defense, though females may spend slightly more time at the nest. Typically, South American Terns raise one brood per year, though favorable conditions may allow for a second.

South American Tern Sounds and Vocalizations

The South American Tern's vocalizations are varied, with a primary song consisting of a rapid series of high-pitched notes, often described as a 'kree-kree-kree' sound. This song serves multiple functions, including mate attraction and territory establishment.

Their calls are sharp and carry well over the ocean, essential for communication within noisy colonies. Alarm calls are harsher and more urgent, alerting others to potential threats. During dawn, their vocal activity increases, with a chorus that signals the start of the day's activities.

Compared to other terns, their calls are slightly lower in pitch but maintain a similar rhythmic quality, aiding in species identification for experienced birders.

Interesting Facts About the South American Tern

1The South American Tern can dive from heights of up to 10 meters, showcasing incredible agility and precision in catching prey.
2Despite their graceful appearance, South American Terns are aggressive defenders of their nests, often mobbing predators like gulls and skuas.
3During the non-breeding season, some South American Terns travel over 2,000 kilometers from their breeding grounds.
4South American Terns have specialized salt glands that allow them to drink seawater, a necessity given their marine lifestyle.
5They can live up to 20 years in the wild, a testament to their adaptability and survival skills in harsh coastal environments.
6Their sharp, high-pitched calls can be heard up to half a kilometer away, crucial for communication within noisy colonies.
7South American Terns often engage in kleptoparasitism, stealing prey from other seabirds in competitive foraging areas.

South American Tern Conservation Status

The IUCN currently lists the South American Tern as a species of Least Concern, indicating stable population levels. However, localized threats exist, primarily due to habitat disturbance and pollution. Coastal development and human activity can disrupt nesting sites, while oil spills and plastic pollution pose significant threats to their marine environments. Conservation efforts focus on protecting key breeding sites and reducing human impact on these sensitive areas. Historically, populations have fluctuated with changes in fish availability and environmental conditions. Ongoing monitoring and habitat preservation are crucial for ensuring the continued stability of this species.