The bottleneck effect
The Jaffna Peninsula, at the northern tip of Sri Lanka, is connected to the rest of the island by a narrow isthmus of land called Elephant Pass. This geography creates a natural chokepoint for migratory birds. It sits on the Central Asian Flyway, a major aerial route that extends from Siberia and northern Europe through Asia to its southernmost destinations. Millions of birds, from over 200 species, travel this path to overwinter in the warmer climate of Sri Lanka.
This narrow passage funnels an immense volume of avian traffic into a concentrated stream. The phenomenon is especially pronounced because the birds are crossing the Palk Strait from the Indian subcontinent, and Elephant Pass is a primary entry point to the island's wetlands and lagoons. The sheer density of birds passing through this corridor makes it a major migratory bottlenecks in Asia. Until recently, the full scale of this migration was difficult to quantify. The installation of specialized radar systems tracks this mass movement, even after dark when most songbirds travel.
Radar ornithology in action
Scientists use repurposed weather or surveillance radar to study the patterns of bird migration, a field known as radar ornithology. These systems emit electromagnetic waves and interpret the signals that bounce back, detecting the size, speed, and density of objects in the airspace. By filtering out weather-related signals, researchers can map bird movements. The radar can track individual flocks and provide data on flight altitude and speed, revealing how birds use wind currents to their advantage.
This technology is particularly effective for monitoring nocturnal migrations, which are nearly impossible to observe otherwise. Data from these systems helps conservationists understand which habitats are most important for birds to rest and feed. This information is used to assess the potential impact of infrastructure like wind turbines, which can pose a collision risk. Research groups like the Field Ornithology Group of Sri Lanka (FOGSL) use this data, along with traditional bird ringing and satellite tagging, to build a comprehensive understanding of migratory routes and protect important stopover sites. Among the many species observed are the Bar-tailed Godwit (Limosa lapponica), Curlew Sandpiper (Calidris ferruginea), Northern Pintail (Anas acuta), and Garganey (Spatula querquedula).