Common Tern (Sterna hirundo) – A Detailed Study
The Common Tern (Sterna hirundo) is a highly migratory seabird that belongs to the family Laridae. It is widely distributed across coastal regions, estuaries, and inland water bodies, playing a crucial role in the aquatic ecosystem. Below is a comprehensive analysis of its taxonomy, morphology, habitat, behavioral traits, and its specific presence in the Sundarban region.
Taxonomy
- Kingdom: Animalia
- Phylum: Chordata
- Class: Aves
- Order: Charadriiformes
- Family: Laridae
- Genus: Sterna
- Species: Sterna hirundo
Morphology (Physical Characteristics)
- Size: Medium-sized tern, with a body length ranging from 31–35 cm, a wingspan of 77–98 cm, and a weight of 110–140 g.
- Plumage:
- Breeding Season: White underparts, light gray upperparts, and a distinct black cap on the head.
- Non-Breeding Season: The black cap fades to a speckled pattern, and the bill becomes darker.
- Bill: Slender, sharp, and typically orange-red with a black tip (more prominent during breeding).
- Legs: Short and orange-red in color.
- Wings: Long, pointed, and streamlined for efficient flight.
- Tail: Deeply forked, giving it an elegant appearance in flight.
Habitat and Distribution
The Common Tern is found in a variety of aquatic habitats across the world, including:
- Coastal regions, estuaries, and riverbanks
- Lakes, reservoirs, and large inland water bodies
- Salt marshes and mudflats
- Sandy and rocky islands used for breeding
This species breeds mainly in temperate and subarctic regions of North America, Europe, and Asia, migrating to tropical and subtropical zones during winter.
Feeding Behavior
- Diet: Primarily feeds on small fish, crustaceans, and aquatic insects. Occasionally consumes mollusks and other marine organisms.
- Hunting Technique: Uses an aerial plunge-diving technique, hovering before swiftly diving into the water to catch prey.
- Foraging Areas: Coastal waters, tidal creeks, and freshwater lakes where fish are abundant.
Breeding and Nesting
- Breeding Season: Typically April to July, depending on the location.
- Nesting Sites:
- Prefers sandy beaches, dunes, or islands with sparse vegetation.
- Nests are simple scrapes on the ground, sometimes lined with pebbles or shells.
- Eggs: Lays 2–4 eggs, speckled for camouflage.
- Incubation: Both parents incubate for 21–25 days.
- Parental Care: Chicks are semi-precocial, meaning they hatch with open eyes and some down feathers but rely on parents for feeding and protection.
Migration and Seasonal Movements
The Common Tern is a long-distance migrant, wintering in warmer tropical and subtropical regions. It follows coastal migration routes and is known for its strong fidelity to breeding and wintering sites.
- Migratory Routes:
- European populations migrate to West Africa and the Indian subcontinent.
- North American populations winter in South America.
- Asian populations move to Southeast Asia and Australia.
Special Features and Adaptations
- Efficient Flight: Long wings enable it to glide effortlessly over water and cover vast distances during migration.
- Aerial Agility: Performs sharp turns and hovering before diving into water for prey.
- Distinct Vocalization: High-pitched calls such as “kirrik” or “kree” are used for communication, particularly during breeding and feeding.
- Mimicry and Defense: Often mobs predators like gulls and crows by attacking in coordinated groups.
Presence of Common Tern (Sterna hirundo) in Sundarban
The Sundarban region, being the largest mangrove forest in the world, provides an important wintering and migratory stopover site for the Common Tern. Here’s an analysis of its presence in the region:
Occurrence in Sundarban
- Found seasonally in the coastal and estuarine waters of the Sundarban, primarily from September to April.
- Uses the mudflats, sandbars, tidal creeks, and estuarine waters as foraging grounds.
- Observed in mixed flocks with other terns such as the Little Tern (Sternula albifrons) and Caspian Tern (Hydroprogne caspia).
Importance in Sundarban’s Ecosystem
- Indicator of a healthy aquatic ecosystem: The presence of Common Terns reflects rich fish populations in the waters of the Sundarban.
- Controls fish populations: By feeding on small fish, it helps maintain ecological balance in the estuarine ecosystem.
- Supports biodiversity: Shares habitat with other waterbirds like egrets, herons, and kingfishers.
Conservation Status and Threats in Sundarban
- IUCN Red List Status: Least Concern (LC), but populations face local threats.
- Threats in Sundarban:
- Habitat loss due to rising sea levels and coastal erosion.
- Disturbance from fishing activities and human encroachment.
- Pollution from plastic waste and industrial runoff affecting water quality.
Conservation Efforts in Sundarban
- Protection under the Sundarban Biosphere Reserve and Ramsar Site designation.
- Local awareness programs to minimize human disturbance.
- Regular monitoring by ornithologists and conservationists to assess migratory patterns.
Conclusion
The Common Tern (Sterna hirundo) is a fascinating migratory seabird, widely recognized for its agile flight, plunge-diving skills, and strong migratory instincts. In the Sundarban, it plays a crucial ecological role as an indicator of estuarine health and fish abundance. While it is not currently endangered, continued conservation efforts are necessary to mitigate habitat degradation and pollution threats in this fragile mangrove ecosystem.