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10 Fascinating Facts About the Bridled Tern

The Bridled Tern, scientifically known as Onychoprion anaethetus, is a seabird that belongs to the tern family. This breathtaking bird can be found in tropical and subtropical oceans across the globe.

This article delves into the features that set the Bridled Tern apart and how to identify it in the wild. It also covers the various plumages and molts experienced by the bird.

Identification

Field Identification

The Bridled Tern has a unique appearance compared to other birds in its family. It is about 33cm in length, and its wingspan ranges from 69 to 78cm.

It has a white body, black cap, and a black band that goes through the eyes and over the nape. Its bill is yellow with a black tip, and its feet are black.

Its upper wings appear silver-gray, while the lower wings are white.

Similar Species

It is crucial to differentiate the Bridled Tern from other birds in its family. The Lesser Crested Tern resembles the Bridled Tern, but the latter has a narrower black eye-line with an extra black line that curves below the eyes.

The Roseate Tern has a similar body size, but its wings and body are pale gray, with a blacker cap. The Sooty Tern and the Black-naped Tern both have completely black wings with white on their forehead extending past the eye-line.

Plumages

Bridled Terns have a breeding and non-breeding plumage. The breeding plumage is characterized by a black band that extends from the eyes to the nape, a white forehead, and a grayish silver color on the upperparts of the wings.

During the non-breeding season, their bill is darker, and the black band on the head that extends from the eyes to the nape is less prominent, often appearing grayish.

Molts

Molting is a natural process that birds undergo, shedding and replacing their feathers seasonally. The Bridled Tern goes through basic, alternate, and juvenile molts.

The basic molt occurs in the non-breeding season, where birds replace all their feathers for newer ones. The alternate molt results in birds growing distinctive plumage in their breeding season.

The Juvenile molt occurs in hatchlings when they are between 20-40 days, whereby they replace their down feathers with new juvenile feathers.

Conclusion

The Bridled Tern is a remarkable bird species with unique physical features. It is essential to identify this bird correctly to differentiate it from other tern birds.

Additionally, understanding the variations in its plumage and molting process further solidifies one’s knowledge of this bird. Take time to spot a Bridled Tern, and you will appreciate it even more.

Systematics History

The Bridled Tern (Onychoprion anaethetus) belongs to the tern family. Initially, the species belonged to the Sterna genus, but it was later discovered that there was more than one species in that genus, and a new genus, Onychoprion, was created to accommodate the Bridled Tern and other species that exhibited differences from the rest of the genus Sterna.

The genus name, Onychoprion, comes from the Greek words, “onyx” and “prion,” meaning talon and saw, respectively, which refers to the sharp claws on the bird’s feet.

Geographic Variation

The Bridled Tern is widely distributed, occupying the tropical and subtropical oceans of the world. These oceans include the Atlantic, Pacific, and Indian Oceans.

The bird is a common sight in the Caribbean, Hawaii, the Red Sea, and the coast of Australia.

Subspecies

There are currently eleven subspecies recognized for the Bridled Tern. These subspecies are distributed across the vast geographic range of the bird, and while they may look similar, they display some differences in physical appearance.

The subspecies are:

1. O.

a. melanopterus – Christmas Island;

2.

O. a.

antarcticus – Antarctica and the Southern Ocean;

3. O.

a. luctuosus – tropical Indian Ocean islands;

4.

O. a.

nubilosus – subtropical Indian Ocean;

5. O.

a. fuliginosus – western and central Pacific islands;

6.

O. a.

serratus – central and eastern Pacific islands;

7. O.

a. anaethetus – Indian and Pacific oceans;

8.

O. a.

melanopterus – northwestern Australia;

9. O.

a. ridgwayi – Caribbean and Atlantic Ocean islands;

10.

O. a.

onychoprion Red sea, Persian Gulf and Arabian sea, and;

11. O.

a. insularum – Hawaiian Islands.

Related Species

The Bridled Tern shares some similarities with other birds in the tern family. The Little Tern (Sternula albifrons) and the Saunders’s Tern (Sternula saundersi) are closely related species that resemble the Bridled Tern.

The Little Tern is smaller and has a yellow bill, while the Saunders’s Tern is larger and has a black tip on its bill.

Historical Changes to Distribution

The distribution pattern of the Bridled Tern has shown some changes over time as a result of human activity. For instance, the introduction of invasive predators to some islands has affected the bird’s population as they feed on their eggs and young ones.

Studies have shown that the Bridled Tern was present on islands in the West Indies, including the Bahamas and Florida, in the early 1800s. In the mid-20th century, the species began to decrease in numbers due to hunting and predation by cats and rats.

The situation became so dire that by the 1950s, the Bridled Tern was deemed rare in the Bahamas. However, measures to control the invasive predators and hunting have helped to restore the Bridled Tern’s population in the region.

In Hawaii, the Bridled Tern was introduced to Laysan Island in the 1920s to reproduce and protect the bird population. Unfortunately, the bird faced competition from the Laysan Albatross (Phoebastria immutabilis) for nesting sites and food, which saw their numbers decline despite conservation efforts.

However, after the removal of the albatross in the 1990s, the Bridled Tern population on Laysan Island began to recover. Human activities have also led to the creation of new habitats for the Bridled Tern.

For instance, in the Middle East, the building of artificial islands and the dredging of ports have created habitats for the bird, expanding its geographical range.

Conclusion

The Bridled Tern is an incredibly adaptable species that has been able to survive changes in its distribution pattern over the years. While human activities have made some subspecies more vulnerable than others, conservation measures have helped to restore their numbers in some areas.

Research on the bird and other related species will help improve its conservation and management, ensuring that populations remain sustainable in the long run.

Habitat

The Bridled Tern is a seabird that frequents tropical and subtropical oceans. Usually, they nest on coral reefs, low islands, or rocks, often near other seabirds such as noddies, boobies, and frigatebirds.

Breeding colonies of the Bridled Tern are often located in warm regions like the Caribbean, where temperatures remain above 20C throughout the year. Other breeding sites are in the Red Sea and the Indian and Pacific Oceans.

Movements and Migration

Bridled Terns are typically non-migratory. However, individuals that breed on the northern edge of their range may move closer to the equator during non-breeding periods to avoid cooler temperatures.

Some populations, such as the subspecies O. a.

antarcticus, are migratory, breeding in the Sub-Antarctic islands in the summer and migrating to the north at winter. These birds are believed to travel thousands of kilometers and follow ocean currents for extended periods, often resting on sea driftwood and other floating debris.

Bridled Terns are known to disperse across broad areas, often following planktonic blooms. These birds typically fly low over the ocean, typically less than two meters above the surface of the water, using their light and narrow wings to glide long distances with minimal energy consumption.

When weather conditions allow, this species forms large flocks and undergoes group foraging. This technique allows individual birds to save energy by taking advantage of the feeding patterns of other individuals in the flock.

Breeding Behavior

The Bridled Tern is monogamous, and once paired, they usually remain together for life. During the breeding season, males engage in courtship displays, which involve aerial stunts, such as high-speed dives and fluttering their wings, while simultaneously calling out to potential mates.

Males will also offer potential mates small gifts, such as small fish or pieces of seaweed, as a token of their affection. Bridled Terns breed in large colonies, and both partners work together to build a nest that they fix onto rocky crevices.

The nests are tiny and are made of bits of coral, seaweed, or other debris. The female lays one or two eggs, which they take turns to incubate for about three weeks.

Chicks hatch with whitish-grey down feathers, and both parents take turns to feed the young birds by regurgitating fish, squid, or crustaceans into their beaks. Juvenile Bridled Terns fledge and fly independently between 30 to 45 days after hatching, and they remain reliant on their parents for food.

The parents will continue to care for their young ones until they are able to feed and fend for themselves.

Conclusion

The Bridled Tern is a fascinating seabird that occupies tropical and subtropical oceans of the world. While some aspects of their behavior patterns are well-understood, there is a need for continued research to improve our understanding of this species fully.

Collaborative conservation efforts and sustained management aimed at controlling invasive predators and restoring destroyed habitats will be necessary to ensure the bird’s survival.

Diet and Foraging

Feeding

The Bridled Tern feeds on small fish, squid, and crustaceans, which they capture while foraging over water bodies near their breeding colonies. They hunt by hovering over the water and diving to grab their prey.

Bridled Terns feed while on the wing, which means they rarely alight on the water. They are proficient fliers and can maintain high speeds to keep up with large schools of prey.

Diet

The Bridled Tern’s diet varies depending on the availability of prey in their environment. Studies have found that the species prey primarily on fish, such as flying fish, goggle-eye, sardines, anchovy, and herring.

However, the availability of these fish may vary across breeding sites and during the breeding season. In some cases, Bridled Terns have been seen feeding on insects and other harmful crustaceans that pose a threat to the stability of their ecological system.

Metabolism and Temperature Regulation

Bridled Terns have evolved unique mechanisms to regulate their body temperature. They have specialized glands located in their eyes that eliminate salt from their bodies and allow them to drink saltwater.

They store the excess salt in a supraorbital gland located just above their eyes and excrete it through the beak in the form of a concentrated salt solution. This adaptation enables them to survive in high-salinity environments that might otherwise be unlivable due to a lack of freshwater.

Sounds and Vocal Behavior

Vocalization

Bridled Terns produce a variety of calls, which serve various purposes, including communication during courtship, competition, and parenting. The birds produce soft, whistling calls when communicating with their siblings or mates.

They produce a harsh cackling sound when threatened or when their nests are being tampered with. During courtship, the birds produce a series of shrill and rapid whistling sounds that are intended to woo potential partners.

Studies have found that the birds’ calls differ depending on whether they are on the ground or in the air. When on the ground, the birds produce a lower pitch, whereas when flying, the calls are much higher in frequency.

The birds also change their vocalization during different stages of the breeding season. During the onset of breeding, their calls have been observed to be soft and more musical.

However, the tone changes, becoming more aggressive as the breeding season progresses. Bridled Terns utilize their vocalizations mainly in communication.

The quality of their calls can reveal vital information about their status and surroundings to other Bridled Terns, affecting their interactions with other species. Also, the passage of information in breeding colonies ensures successful breeding since, in groups, collective breeding protection, and the provision of safety against predators are maximized.

Conclusion

The Bridled Tern is an incredible species that has evolved to thrive in unique environments. The adaptation of specialized glands to drink saltwater highlights their adaptive nature.

Their unique vocalization and calls possess essential communication in breeding, protecting, and warning against potential predators, while also creating unique features in their avian lives. These efforts highlight how they continue evolving to fit the conditions of their environment, even under changing conditions.

Behavior

Locomotion

Bridled Terns are incredibly skillful at flight and are highly adapted to aerial life. They have long and narrow wings that help them glide through the air effortlessly.

The birds’ wings are brown and sturdy, and they have a wingspan of 69-78cm. Bridled Terns are excellent aerial feeders, and they can glide through the air, catching prey while on the wing effortlessly.

They can stay in the air for extended periods with minimal flapping, thanks to their streamlined body design.

Self Maintenance

Bridled Terns are fastidious birds that take care to maintain their feathers and keep their bodies clean. They will frequently dunk themselves in the ocean or dive into water sources to wet their feathers and preen them to remove dirt and excess oil.

Their waterproof feathers keep their feathers from getting waterlogged while flying or hunting in wet environments.

Agonistic Behavior

Bridled Terns are relatively passive birds, but they may exhibit agonistic behavior when another individual or predator poses a threat. They will often flock around the nest site when a potential threat, including other birds, humans, or predators, comes too close.

Bridled Terns may also use aggressive displays, such as pecking, diving, or calling, to repel predators or potential intruders from their nesting territories.

Sexual Behavior

Bridled Terns are monogamous birds that will form long-term pair bonds with a mate. During the breeding season, males will engage in courtship displays, such as aerial acrobatics and bringing small gifts to present to females.

After pair bonding, Bridled Terns will build a small nest of shells, pebbles, and other debris, typically wedged into crevices along rocks or on coral islands.

Breeding

Bridled Terns typically breed in large rookeries on coral islands, rocks, and sandy beaches. Like most terns, they form dense breeding colonies that provide safety from predators and improve the chances of successful breeding in pairs.

Bridled Terns lay one to two eggs per breeding season, incubate them for three weeks, and care for the chicks until they are old enough to fly and hunt on their own.

Demography and Populations

The global population of Bridled Terns is estimated to be around three to four million birds. The population was relatively stable until the 20th century, when several human-made factors started to impact it negatively.

The introduction of invasive predators to islands where the birds breed has led to a significant decline in their numbers. Management strategies have been put in place to help conserve Bridled Tern populations, focusing on predator control and restoring degraded habitats.

Monitoring programs are also in place to track population trends and determine the effectiveness of conservation efforts. The conservation status of the Bridled Tern is currently listed as a species of least concern, although subspecies like O.

a. antarcticus are endangered.

The Bridled Tern is an incredible seabird species that has captured the interest of bird enthusiasts worldwide. The bird is unique in various ways, from its aerodynamic flying style to its specialized glands that allow it to drink saltwater and survive in high-salinity environments.

Further, its vocalizations, breeding, and foraging behavior, including its migrations, have helped to make them one of the most wonderfully adapted birds in their ecological community. Nonetheless, current conservation and management efforts are necessary, especially when it comes to fighting predators and restoring habitats to maintain sustainable populations of Bridled Terns.

Studying the Bridled Tern, and other bird species, is essential as it helps us understand both the remarkable ways species adapt to their environment and the pivotal role conservation plays to ensure the perpetuity of these unique lifeforms.

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