South American Terns nesting on Cardos Island

South American Terns: Unveiling the Secrets of Their Coastal Breeding Success

Lena Kashyap in Science & Nature December 2025 • 4 min read.

"Discover how strategic nesting sites and low predator pressure contribute to the reproductive success of South American Terns in Brazil."

The reproductive success of coastal seabirds is a delicate balance, influenced by climate, food availability, predation, and human impact. Marine terns, often more specialized than other coastal species, face unique vulnerabilities in their breeding and foraging grounds.

While the breeding biology of terns in the northern hemisphere is well-documented, less is known about their southern counterparts. The South American Tern (Sterna hirundinacea), a migratory seabird inhabiting the Pacific and Atlantic coasts of South America, offers a fascinating case study.

This article delves into the reproductive success of South American Terns on Cardos Island, Florianópolis, Brazil, drawing from research conducted over three breeding seasons (2003, 2005, and 2006) to shed light on the factors that contribute to their survival.

Cardos Island: A Haven for South American Terns

South American Terns nesting on Cardos Island

Cardos Island, located in Santa Catarina, Brazil, provides a unique environment for South American Terns. The island's thick vegetation and rocky terrain offer nesting sites, while its surrounding waters provide feeding grounds. Researchers conducted daily observations on the island during the breeding seasons of 2003, 2005, and 2006.

The study revealed valuable insights into the terns' breeding habits:

Colony Formation: Colonies typically formed in mid-May in 2003 and early April in subsequent years.
Nest Numbers: The total number of nests varied, ranging from 1,852 in 2006 to 2,486 in 2005.
Hatching Success: Hatching success was highest in 2006 (76.39%) but significantly lower in 2005 (41.1%), likely due to increased predation.

Predation played a significant role in reproductive success. Hawks (Caracara plancus), lizards (Tupinambis merianae), and black vultures (Coragyps atratus) were identified as predators of eggs and chicks. However, the relatively low presence of Kelp Gulls (Larus dominicanus), a major seabird predator along the Brazilian coast, contributed to the terns' breeding success.

Conservation Implications and Future Directions

The consistent use of Cardos Island as a breeding site underscores its importance for the conservation of South American Terns. Understanding the factors that contribute to their reproductive success is crucial for developing effective conservation strategies.

Further research is needed to explore the long-term trends in tern populations, the impact of climate change on their breeding habitats, and the effectiveness of different conservation interventions. Continuous monitoring of breeding colonies and predator populations is essential for ensuring the long-term survival of these fascinating seabirds.

By protecting key breeding sites like Cardos Island and mitigating threats such as predation and human disturbance, we can help safeguard the future of South American Terns and maintain the biodiversity of coastal ecosystems.

About this Article -

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This article is based on research published under:

DOI-LINK: 10.1590/s0001-37652012005000020, Alternate LINK

Title: Reproductive Success Of South American Terns (Sterna Hirundinacea) From Cardos Islands, Florianópolis, Sc, Brazil

Subject: Multidisciplinary

Journal: Anais da Academia Brasileira de Ciências

Publisher: FapUNIFESP (SciELO)

Authors: Hélio A.A. Fracasso, Joaquim O. Branco

Published: 2012-04-12

Everything You Need To Know

What role do nesting sites play in the reproductive success of South American Terns?

The South American Tern's reproductive success hinges on strategic nesting sites that provide protection from predators and the elements. The Cardos Island study highlights the importance of thick vegetation and rocky terrain for nest construction. The consistent return to Cardos Island over multiple breeding seasons underscores the suitability and reliability of these nesting sites for South American Terns.

Which animals pose a threat to South American Tern eggs and chicks, and why is the presence of Kelp Gulls important?

Predation significantly affects South American Tern reproductive success. Hawks (Caracara plancus), lizards (Tupinambis merianae), and black vultures (Coragyps atratus) prey on eggs and chicks. The relatively low presence of Kelp Gulls (Larus dominicanus), which are major predators of other seabirds, on Cardos Island is a contributing factor to the South American Tern's breeding success.

What did studies reveal about South American Tern colony formation, nest numbers, and hatching success?

The study on Cardos Island spanned three breeding seasons (2003, 2005, and 2006). Colony formation varied, occurring in mid-May in 2003 and early April in subsequent years. Nest numbers ranged from 1,852 in 2006 to 2,486 in 2005. Hatching success also fluctuated, reaching 76.39% in 2006 but dropping to 41.1% in 2005, likely due to increased predation. These variations demonstrate the dynamic nature of breeding success and the impact of environmental factors.

Why is Cardos Island important for the conservation of South American Terns, and what strategies can improve their survival?

Cardos Island's role as a consistent breeding site for South American Terns emphasizes its conservation value. The island provides a haven with suitable nesting habitats and relatively low predator pressure compared to other coastal areas. Protecting Cardos Island and similar locations is crucial for ensuring the long-term survival of South American Tern populations. Conservation strategies could focus on minimizing human disturbance during breeding seasons and managing predator populations to further enhance breeding success. More research is required to address other factors not detailed, such as food source availibility and contamination effects.

Where are South American Terns located, and what makes studying their reproductive success important for conservation?

South American Terns (Sterna hirundinacea) are migratory seabirds found along the Pacific and Atlantic coasts of South America. These birds face unique vulnerabilities in their breeding and foraging grounds, making them valuable subjects for ecological research. The species' reproductive success is of particular interest because it is a key indicator of population health and the effectiveness of conservation efforts. Studying South American Terns can provide insights applicable to other seabird species facing similar challenges from habitat loss, climate change, and pollution.