Satellite-tagged bar-headed geese here revealed they cross the Himalayas at 7,000m in eight hours, the highest bird migration known. They breathe air with 10% normal oxygen and survive. The impossible commute.
Tisha Mukherjee, CC BY-SA 4.0, via Wikimedia Commons
The high-altitude engine
Each winter, Chilika Lake hosts one of nature's most extreme athletes: the bar-headed goose (Anser indicus). Satellite tracking studies conducted here revealed the full scope of their astounding migration. Birds tagged at Chilika leave their sea-level wintering grounds and, in a single flight, ascend the Himalayas. The northward crossing takes as little as eight hours, with the geese reaching altitudes between 4,000 and 6,000 meters. One tracked bird was recorded at a staggering 7,290 meters. At these heights, oxygen levels are so low that most humans would suffer from severe altitude sickness.
The geese accomplish this feat without the weeks of acclimatization that human climbers require. They fly through the mountain valleys, often at night when the air is colder and denser, which provides more lift and makes flapping flight more efficient. This shows they are not riding tailwinds and are actively powering their way over the world's highest mountain range in a sustained, energy-intensive climb.
A suite of biological upgrades
The bar-headed goose's ability to fly in severely hypoxic (low-oxygen) conditions stems from a unique set of physiological adaptations. Their hemoglobin, the protein in red blood cells that carries oxygen, has a higher affinity for O2, a trait attributed to a single amino acid point mutation. This allows their blood to load oxygen far more efficiently in the thin mountain air.
Their physiology is optimized for oxygen delivery. Compared to low-altitude geese, they have proportionally larger lungs and can hyperventilate at up to seven times their resting rate without ill effects. Their flight muscles contain a higher density of capillaries, and the mitochondria—the cells' powerhouses are located closer to the cell membranes. This arrangement shortens the diffusion distance for oxygen to travel from the blood to the muscles that need it. The left ventricle of the heart also possesses more capillaries, ensuring the cardiac muscle itself remains well-oxygenated to maintain a high heart rate during extreme exertion. These adaptations combine to create a biological engine capable of performing one of the most challenging migrations on Earth.
Chilika: a continental hub
Chilika Lake is the largest brackish water lagoon in Asia, covering over 1,100 square kilometers. Its status as a Ramsar Wetland of International Importance, designated in 1981, recognizes its critical role as a wintering ground for migratory birds using the East Asia-Australasia Flyway.
During the peak season, the lake's bird population exceeds one million individuals. The annual bird census for 2026 recorded 1,132,200 birds from 196 different species. The most numerous species was the Northern Pintail, with a count of 218,993. The center of this avian activity is the Nalabana Bird Sanctuary, a 15.53 square kilometer island in the lake. The island, whose name means "weed-covered island" in the Odia language, becomes completely submerged during the monsoon season and re-emerges as water levels recede, creating vast, nutrient-rich mudflats that attract hundreds of thousands of birds.
💡Fun Facts
A single point mutation in the hemoglobin of bar-headed geese is why they can absorb enough oxygen to fly at extreme altitudes.
The Nalabana Bird Sanctuary, the core area for birds in Chilika, completely disappears underwater during the monsoon season each year.
The January 2026 bird census at Chilika counted 1,132,200 birds, with 397,587 of them concentrated in the Nalabana sanctuary alone.
Bar-headed geese can hyperventilate up to seven times their normal resting rate to maximize oxygen intake in thin air.
The lake is accessible year-round. Boat services for bird and dolphin watching typically run from early morning (around 6 AM) to late afternoon (around 6 PM). The best birdwatching is from November to February.
Admission
Access to the lake is free, but boat hire is necessary for exploration. Costs vary based on the type of boat and duration, typically ranging from ₹1,000–₹1,500 per boat.
Accessibility
Access to the main birdwatching and dolphin-spotting areas is via motorboat from jetties at Satapada, Barkul, or Rambha. The areas around the jetties are generally flat.
