Bridled Tern (Onychoprion anaethetus) – A Comprehensive Study

Taxonomy & Classification

• Kingdom: Animalia
• Phylum: Chordata
• Class: Aves
• Order: Charadriiformes
• Family: Laridae
• Genus: Onychoprion
• Species: Onychoprion anaethetus

The Bridled Tern (Onychoprion anaethetus) is a member of the tern family Laridae, known for its graceful flight and distinctively marked plumage. It is a medium-sized seabird widely distributed across tropical and subtropical waters.

Morphology & Physical Description

• Size: 30–32 cm in length
• Wingspan: 77–81 cm
• Weight: 140–190 grams
• Plumage: Upperparts are dark grey to black, while underparts are white.
• Head Features: The black cap extends from the bill to the nape, with a distinctive white forehead and eyebrow-like streaks.
• Beak: Slender, black, and slightly hooked at the tip.
• Legs & Feet: Short and black, adapted for perching rather than walking.
• Tail: Forked, adding agility during flight.

Habitat & Distribution

The Bridled Tern is a marine bird, preferring warm tropical and subtropical oceans, islands, and coastal regions. It is commonly found:

• Along the coastlines of the Indian Ocean and the Pacific Ocean
• Around coral atolls, rocky islets, and mangrove-rich shorelines
• Breeding mainly on offshore islands and undisturbed coastal sites

Presence in the Sundarban

The Sundarban, the largest tidal halophytic mangrove ecosystem, provides an essential habitat for many bird species, including the Bridled Tern. This species is primarily seen in:

• Coastal and estuarine zones of the Sundarban
• Mangrove-lined riverbanks and islets
• Breeding colonies on isolated sandbanks and offshore islands

In the Sundarban Biosphere Reserve, the Bridled Tern is not a permanent resident but a seasonal visitor, particularly during the post-monsoon and winter seasons, when food availability is high.

Feeding Behavior & Diet

The Bridled Tern is a pelagic feeder, relying primarily on:

• Small fish (anchovies, sardines)
• Squid and other cephalopods
• Crustaceans and marine invertebrates
• Occasionally, it follows fishing boats to scavenge scraps.

Foraging Strategy

• Hovering over water and plunge-diving to capture prey
• Prefers shallow coastal waters rather than deep ocean zones
• Often seen in mixed-species flocks with other seabirds

Breeding & Nesting

• Breeding Season: Typically April to August, varying with location
• Nest Sites: Prefers isolated, predator-free rocky outcrops, sandbanks, and coastal cliffs
• Nesting Type: Minimalist – lays eggs in shallow scrapes in sand, rock crevices, or beneath vegetation
• Eggs: Usually one egg per clutch, camouflaged with mottled brown and beige coloration
• Parental Care: Both parents share incubation (about 28-30 days) and chick-rearing

Vocalization & Communication

The Bridled Tern has distinct calls, including:

• Sharp “kree-kree” or “kirri-kirri” sounds, often heard during flight
• Low growls and soft whistles during courtship and feeding interactions

Special Adaptations

1. Long-Distance Migration: Can travel thousands of kilometers between breeding and feeding grounds.
2. Efficient Flight Mechanism: Long wings allow energy-efficient gliding over vast oceanic distances.
3. Camouflaged Nesting: Helps protect eggs from predators like crabs and gulls.
4. Salt Gland Adaptation: Excretes excess salt, allowing survival in high-salinity environments like mangroves and seawater coasts.

Threats & Conservation Status

• Predation by invasive species (rats, cats, and larger birds) on breeding islands
• Coastal habitat destruction due to tourism, pollution, and industrial expansion
• Climate change effects like rising sea levels and storm surges
• Oil spills and marine plastic pollution, affecting foraging efficiency

IUCN Status: Least Concern (LC)

Despite localized threats, the global Bridled Tern population remains stable due to its wide range and adaptability. However, continued mangrove conservation efforts in the Sundarban are crucial for maintaining its presence in the region.

Conclusion

The Bridled Tern (Onychoprion anaethetus) is an elegant seabird, thriving in warm coastal ecosystems like the Sundarban. Though not a resident species, its seasonal presence highlights the region’s rich biodiversity. Protecting coastal and marine habitats is essential to ensure its continued survival in this fragile ecosystem.