The Caspian tern (Hydroprogne caspia) is the largest tern in the world. Despite its large size and formidable appearance, this seabird has a number of weaknesses that can make it vulnerable in certain situations.
Small population size
One of the main weaknesses of the Caspian tern is its relatively small global population size. The total number of mature Caspian terns in the world is estimated to be somewhere between 150,000-200,000 individuals. While this may seem reasonably large, it is small compared to many other bird species. As a result, the Caspian tern is classified as “Near Threatened” on the IUCN Red List of Threatened Species.
A small population size makes a species more vulnerable to threats such as habitat loss, introduction of invasive species, pollution, disease and severe weather events. Even a localized disaster, such as an oil spill, could potentially wipe out a significant percentage of the total Caspian tern population. With fewer total numbers, it can be harder for the species to recover after such an event.
Dependence on limited nesting habitat
Caspian terns are colonial nesters that gather in large groups numbering in the hundreds or thousands of pairs to breed. However, suitable nesting habitat is limited for this species. Caspian terns require undisturbed islands or coastal areas with sandy or gravelly substrates. Competition for nesting space from other seabird species can further limit available habitat.
This dependence on scarce nesting habitats makes Caspian terns vulnerable. Disturbance, development or degradation of their breeding grounds can prevent terns from nesting successfully. Rising sea levels due to climate change are also projected to impact shoreline nesting areas. With limited options, Caspian terns may not have alternative nesting sites available if they are displaced from traditional colonies.
Reliance on vulnerable food sources
Caspian terns are almost entirely piscivorous, feeding mainly on small fish ranging from 3-20 cm in length. Some of their key prey species include Pacific herring, anchovies, smelt and capelin. However, many of these fish stocks are under pressure from factors like overfishing, pollution and climate change.
Declines in food availability can cause issues for Caspian terns during the breeding season, when they need abundant food sources within close flying distance of their colonies. Poor feeding conditions are associated with lower breeding success, as terns may abandon nests or produce fewer chicks when food is scarce. The reliance on fish makes the terns vulnerable if ocean conditions change and critical prey populations shift or crash.
Sensitivity to disturbance
Caspian terns appear fierce, thanks to their crimson bills and imposing size, but they are actually quite sensitive to disturbance. Their preferred nesting habitats on open beaches and islands leave them exposed, and terns will readily abandon nests if approached by humans or predators. Even loud noises, aircraft overflights or fireworks displays can cause mass nest desertion.
This nervous disposition developed because the ground-nesting terns are otherwise very vulnerable while incubating eggs and brooding flightless chicks. Their sensitivity to disturbance is an adaptive tradeoff to avoid predation. However, too much interference prevents Caspian terns from successfully raising young, which over time can lead to population declines.
Exposure to predators
Both eggs and chicks are vulnerable to land-based predators such as foxes, coyotes, bears, raccoons and gulls. Chicks and fledglings may also be taken by eagles and other raptors. Even large gull species will prey on tern eggs and chicks when given the opportunity.
Adult Caspian terns can defend themselves from aerial attacks using their dagger-like bill, but grounded chicks and eggs are largely unprotected. Terns nest in colonies for added protection, yet the presence of potential predators remains a constant threat. Predation pressure is a major factor in nesting success for the species.
Sensitivity to severe weather
The exposed coastal and island habitats favored by nesting Caspian terns offer little protection from extreme weather. Storms, high winds, flooding, freezing temperatures and heat waves can all negatively impact tern colonies.
Intense storms may wash away entire nesting areas along with eggs and chicks. Cold weather and rainfall can lead to hypothermia and mortality of chicks. Hot weather raises the risk of overheating, dehydration and sun exposure to eggs and chicks. Even moderate wind and rain can erode earthen nests and reduce survival. Having little shelter, Caspian terns are quite vulnerable to severe environmental conditions.
Risk of disease
Living in dense colonies puts Caspian terns in close contact and at increased risk of infectious diseases. Avian cholera, avian botulism and avian influenza are some of the most serious disease threats. Outbreaks in tern colonies have caused mass mortality events, including the 2002 botulism outbreak that killed over 10,000 Caspian terns at the Salton Sea in southern California.
The crowded conditions in breeding colonies facilitate rapid disease transmission once a pathogen is introduced. While adult terns may have resistance, unfledged chicks are highly susceptible. Diseases may become more prevalent due to climate change, further threatening colonies. Lack of genetic diversity may also make terns more vulnerable to novel diseases.
Accumulation of environmental contaminants
As fish-eating seabirds, Caspian terns are prone to bioaccumulation of heavy metals like mercury and selenium, as well as synthetic contaminants like PCBs. These substances become concentrated as they move up the food chain to tertiary consumers like terns.
In birds, elevated contaminant loads can cause problems with reproduction, growth and development, hormone regulation and immune function. The small population size of Caspian terns means that impacts on fitness from pollution bioaccumulation could have population-wide effects over time.
Interactions with commercial fisheries
Caspian terns frequently come into conflict with commercial fisheries. Terns are drawn to aquaculture sites and fish farms, where they prey on stocked fish. Their colonies may also become established on shellfish lease sites, negatively impacting operations.
Terns are often viewed as competitors by fishery stakeholders, resulting in disturbance, exclusion or culling of birds at fish farms. Commercial overfishing also depletes the wild fish stocks that terns rely on. The interactions with fisheries pose ongoing risks for Caspian tern populations through much of their range.
Climate change effects
Climate change is projected to have multiple impacts on Caspian terns, as seabirds are highly vulnerable to shifts brought about by global warming. Sea level rise may inundate low-lying nesting sites, while increasing storm activity could damage breeding colonies.
Ocean warming is expected to alter food web dynamics, affecting the fish prey populations that terns depend on during reproduction. Changing wind, precipitation and temperature patterns may also impact breeding habitat suitability and reproductive timing. The long-term consequences of climate change are uncertain but likely to add further stresses to Caspian tern populations.
Anthropogenic threats on migration
Caspian terns undertake long distance migrations spanning thousands of kilometers between their breeding and wintering grounds. These lengthy journeys expose terns to hazards including power line collisions, wind turbine strikes, and hunting or disturbance in staging and wintering areas.
Migratory birds worldwide are facing escalating threats from human infrastructure and activities. The risks are elevated for species like Caspian terns that congregate in large flocks during migration. Even occasional events resulting in high mortality could significantly affect their global population due to its small size.
Lack of coordinated conservation effort
Caspian terns have a broad distribution across North and South America, Europe, Asia, Africa and Australasia. However, conservation efforts for the species are not well coordinated across different countries and regions where terns occur.
More intensive monitoring and protection may be beneficial in certain key breeding colonies that make up a large proportion of the total population. Though Caspian terns remain widespread, a targeted approach to conserve important subpopulations could help strengthen overall population resilience.
Conclusion
In summary, Caspian terns face a range of weaknesses and threats that leave them vulnerable as a species. Their small total population size, dependence on limited nesting habitat, sensitivity to disturbance, and exposure to predators and severe weather events make breeding a challenge. Terns also face risks from climate change, pollution, fishery interactions, disease and anthropogenic threats during migration.
While Caspian terns appear imposing, they face many of the same conservation issues as other seabirds. Ongoing monitoring and protection of key breeding colonies will be important to safeguard Caspian terns into the future. Management should focus on preserving important habitat, reducing disturbance, improving food availability, limiting contaminants and disease, and mitigating climate change impacts where possible.