The need for natural resources, curiosity to know more, and hunger for knowledge has always driven humans to explore and discover the new and unbelievable in every corner of this planet and beyond. One of these exciting options is the world beneath the planet’s surface. Subterranean ecosystems are frequently overlooked and unnoticed because of their inaccessibility, absence of light, and other unknown conditions. Therefore, there is a lack of knowledge about the ecosystems and animals that live in caves. One of the most delicate ecosystems on the planet, caves are home to a wide variety of organisms. Caves are natural underground voids usually divided into three zones: entrance, twilight, and dark. Echolocating birds (those that use reflected sound waves), often mistaken for bats, are one of the most impressive animals living in the dark zones of caves. These birds are small paleotropical swiftlets (Family: Apodidae). The only other birds that echolocate and live in the dark zones of caves are neotropical oilbirds (Family: Steatornithidae).
The Indian subcontinent is home to four of the world’s 39 species of swiftlets: plume-toed swiftlet, Indian swiftlet, Himalayan swiftlet, and edible-nest swiftlet. Swiftlets are part of the Apodidae family, which translates to “footless” in Latin. These birds have weak legs, preventing them from perching. However, their powerful claws allow them to cling to vertical surfaces and ceilings. Swifts and swiftlets built their nests on the walls or ceilings of human-built structures (buildings, bridges, etc.) or caves. Most swiftlets build their nests by binding vegetative materials (leaf, moss, etc.) and their own feathers with saliva (secreted by salivary glands that enlarge during the breeding season). The nest is attached to the cave wall and ceilings with saliva, which, once dry, forms a hard cement (nest cement) to keep the bracket/half-cup shape.
(1) Old Lighthouse and Burnt islands are a part of the Vengurla Rocks archipelago, a group of islands offshore from Sindhudurg, Maharashtra. The largest colony of Indian swiftlets are found here. (2) Swiftlets are found in rocky hill and lowland areas with suitable nesting caves. They rely on echolocation to navigate and locate their nests in the dark.
Cover photo: The Indian swiftlet is a small, brown bird, around 12 cm in length, found in the western India and throughout Sri Lanka. It frequents small rocky offshore islands as well as mainland habitats. The puffiness in this bird’s throat indicates that it is carrying food for its young.
The nests of at least four species of swiftlets are used for commercial purposes; these include the edible-nest swiftlet (Aerodramus fuciphagus), Germain’s swiftlet (Aerodramus germani), black-nest swiftlet (Aerodramus maximus), and the Indian swiftlet (Aerodramus unicolor). The edible-nest swiftlet (Aerodramus fuciphagus), also known as the white-nest swiftlet producer and Germain’s swiftlet, constructs its opaque off-white nests exclusively of saliva. Incredibly, each swiftlet pair uses about 8-10 gm of saliva to construct a nest.
The other two commercially important species use a significant amount of saliva with vegetative matter or feathers to produce the black edible nests. The swiftlet’s ability to produce large quantities of salivary secretion, as evidenced by the presence of salivary glands in the oropharyngeal (mouth and throat) region, makes its nest edible enough to be harvested for “bird’s nest soup,” a multibillion-dollar industry driven by consumer demand.
Swiftlets have adapted quite stunningly to live inside dark underground habitats using echolocation. They do not use echolocation to detect and catch their food. It is believed (but unconfirmed) that they use sonar waves for navigation and finding their nests in the dark. This species uses audible clicks (tik-tik) of frequencies (1-10 kHz) that are within the human auditory range. The systematic counting of these audible clicks produced while echolocating has made it possible for researchers to successfully estimate the swiftlet populations inside caves for several decades.
In the late 1990s, Dr Ravi Sankaran, former Director of the Salim Ali Centre for Ornithology and Natural History (SACON), began efforts to conserve the edible-nest swiftlet of the Andaman and Nicobar Islands. After more than two decades of learning about the species and successful conservation efforts, the survival of this species has been ensured. This exceptional and inspiring journey and the uniqueness of swiftlets motivated us to learn more about the species and work towards their conservation.
Burnt Island is part of Vengurla Rocks archipelago, a group of islands 11-12 km offshore in Sindhudurg, Maharashtra. It is home to the largest Indian swiftlet colony with over 5,000 birds. Other colonies exist in the southern Western Ghats and Sri Lanka.
Burnt Island is so named as it appears burnt from afar because of its blackish rock surface, sparsely covered with dried grass. When we first saw it, part of the island looked like it was snow-covered. With several species of terns that also visit seasonally to breed, this is designated an Important Bird Area. In 2019, we first visited Burnt Island and entered the sea cave to see the black edible nests built by the Indian swiftlets using their own feathers bound with saliva. Swiftlets are social birds that flock together in pairs in large colonies of thousands. As these monogamous birds (a pair is faithful until something happens to one of them) build a nest each season, the number of breeding birds can be roughly calculated from the number of nests found within the caves.
After hunting small aerial insects from morning to evening, swiftlets return to their roosts on the Burnt and Old Lighthouse Islands at dusk. The twilight sky is full of swiftlets, flocking at the entrance of this huge sea cave on Burnt Island, calling and chasing, and then entering the cave as fast as possible to escape any awaiting predator. The large number of swiftlets entering a cave together makes counting impossible.
(1) Indian swiftlets nest colonially in caves, including sea caves and railway tunnels. Their nests are often densely clustered, with just 5-20 cm between them. (2) The self-supporting, bracket-shaped nest built with grass, twigs, moss, and feathers stuck together with saliva weighs around 6–8 gm. Sometimes, the nests are made almost entirely of saliva, lending them an “edible” quality.
Besides the unique nests, their foraging behaviour caught our attention. Every day, swiftlets leave their cave at first light and travel long distances to feed over various ecosystems, including mangroves, plantations, wetlands, and forests. They constantly scan the area for airborne insects. During their long days of foraging, the birds rely heavily on wetlands to drink water. During the breeding season, the birds frequently return with food boluses to feed their young. But as the day draws to a close, before roosting for the night, the swiftlets congregate midair outside the cave, and we get to see hundreds/thousands of them swarm at the entrance of the cave, much like raindrops falling on earth’s surface.
(1) The breeding season of these monogamous birds occurs mainly between March and June in India. (2 & 3) These highly aerial birds have narrow wings and a shallow forked tail that facilitates flight. They display a characteristic crescent shape when in the air.
Indian swiftlets on Burnt Island are an integral part of the cave ecosystem. Protecting swiftlets is essential to preserving a wide range of cave-dwelling species here. In the absence of bats, by providing guano (droppings), swiftlets provide food and energy to other life forms in these oligotrophic (low organic matter) ecosystems. Outside caves, swiftlets are top predators of many insect species (including agricultural pests).
Guano produced by bats and birds, is an essential organic matter resource in cave ecosystems, supporting the survival of many invertebrates. The presence of guano inside caves also contributes to keeping the microclimate stable. Bats and birds in caves are recognised as the umbrella taxa in cave ecosystems. In the absence of bats, swiftlets carry the responsibility.
About the authors
Dr Manchi Shirish S.
Dhanusha Kawalkar
Dhanusha Kawalkar is a budding ornithologist and bio-speleologist with a keen interest in cave-ecology and community based conservation. She is currently a PhD Scholar at SACON and co-founder of Speleological Association of India.
Related Content
