Bird O'clock

Flying Acrobats: Discovering the Fascinating World of Bare-legged Swiftlets

The Bare-legged Swiftlet, or Aerodramus nuditarsus, is a small bird that can be recognized by its bare legs. These tiny birds are known for their fast and agile flights, making them a fascinating sight to behold for bird lovers and enthusiasts.

In this article, we will be discussing this interesting species in more detail, from its identification and similar species to its plumage and molts.


Field Identification

The Bare-legged Swiftlet is a small bird that measures between 8-9cm in length. It has a square tail, a dark brown to blackish upperparts, and a pale rump.

Its underparts are pale grey to white, and it has a short and stubby bill. The species’ most distinguishing feature is its bare legs, which have a pale pink to greyish color.

Similar Species

The Bare-legged Swiftlet is often confused with other members of the swiftlet family. Two species that are often misidentified are the Black-nest Swiftlet and the Germain’s Swiftlet.

The Black-nest Swiftlet has longer wings and a more pointed tail. It also has a slightly larger and more robust body and a shorter bill than the Bare-legged Swiftlet.

The Germain’s Swiftlet, on the other hand, has white underparts instead of pale grey to white and a darker brown upperparts. It also has a longer and more slender bill.


The Bare-legged Swiftlet has a relatively simple plumage, with no significant differences between males and females. The species’ colors are uniform and unremarkable, with a dark brown to blackish upperparts and pale grey to white underparts.


Like other swiftlets, the Bare-legged Swiftlet undergoes an unusual molting process known as the “slow molt.” Instead of dropping their feathers all at once and growing new ones, they gradually molt their feathers and grow new ones at the same time. This means that they always have a mix of old and new feathers, leading to a somewhat scruffy appearance.

In conclusion, the Bare-legged Swiftlet is a unique species of bird with its bare legs being the most striking feature. With its fast and agile flights, it’s no wonder that it is a favorite among bird enthusiasts.

Careful observations, however, are necessary to distinguish it from other similar swiftlet species. Understanding their molting process and plumage changes also provide further insight into the biology and behavior of these fascinating birds.

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Systematics History

The Bare-legged Swiftlet was first described by German naturalist Hermann Schlegel in 1866. Its scientific name, Aerodramus nuditarsus, is derived from the genus Aerodramus, which includes swiftlets, and the Latin words “nudus” and “tarsus,” which means uncovered or bare and legs, respectively.

Geographic Variation

The Bare-legged Swiftlet is found throughout Southeast Asia, including Indonesia, Malaysia, the Philippines, and Thailand, among other countries. There is no significant geographic variation in the species’ appearance, but its distribution varies depending on the subspecies.


There are two recognized subspecies of the Bare-legged Swiftlet:

Aerodramus nuditarsus nuditarsus – This subspecies is found in the central and southern Philippines. Aerodramus nuditarsus sororum – This subspecies is found on the island of Sulawesi, Indonesia.

Related Species

The Bare-legged Swiftlet belongs to the genus Aerodramus, which is part of the Apodidae family, also known as swifts. There are more than 100 species of swifts, and they are found all over the world, except in Antarctica.

The genus Aerodramus, in particular, comprises around 25 species of swiftlets that are mostly found in Southeast Asia.

Historical Changes to Distribution

The distribution of the Bare-legged Swiftlet has undergone significant changes over time, primarily due to human activities such as habitat destruction and urbanization. The species is considered “near-threatened” by the International Union for Conservation of Nature (IUCN) due to the loss of its natural habitat.

In Thailand, for instance, the Bare-legged Swiftlet was previously common but is now rarely seen due to habitat destruction, particularly in the northern provinces. The same trend is observed in the Philippines, where the species’ population is also in decline due to habitat loss and conversion of forests into agricultural lands.

In Malaysia, the Bare-legged Swiftlet population is still stable, and the species is relatively common in urban areas where it often nests in buildings. In Indonesia, the population is also considered stable, with most populations found in protected areas and forested regions.

The Bare-legged Swiftlet plays an essential role in its ecosystem as an insectivore. It feeds on various insects in flight, including flying ants, termites, and beetles.

The species also plays a vital role in pollination, particularly in its natural forest habitat, contributing to the maintenance of several plant species. Unfortunately, the Bare-legged Swiftlet’s ability to pollinate and control insect populations may be affected by habitat destruction.

Therefore, the conservation of the species’ natural habitat is crucial to its survival and to maintaining the ecological balance of its ecosystems. In conclusion, the Bare-legged Swiftlet has undergone significant changes in its distribution due to anthropogenic activities.

Despite being relatively common in some parts of its range, the species remains under threat due to the loss of its natural habitat. It is essential to prioritize habitat conservation measures for the species to ensure its survival and prevent further declines in its population.

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The Bare-legged Swiftlet is a bird species that has adapted to a wide range of habitats, from forests to urban areas. In general, it prefers cooler and more humid environments, particularly in its natural forest habitat.

The species can be found roosting and breeding in caves, buildings, and trees. It constructs its nests mainly with saliva, which hardens into a nest cup.

The entrance of the nest, which faces downward, creates a unique “hourglass” shape. These structures are often found in colonies of hundreds or even thousands, depending on the site’s availability.

In urban areas, the Bare-legged Swiftlet has adapted to nesting in buildings, including high-rise structures such as towers. The species has become a common sight in many cities in Southeast Asia and is even part of the local culture in some areas.

Movements and Migration

The movements and migrations of the Bare-legged Swiftlet are not well understood due to the species’ difficulty in tracking. Like other swiftlets, the Bare-legged Swiftlet is highly mobile and capable of flying great distances.

Its flight is characterized by sudden twists, turns, and aerial acrobatics. The species is known to undertake local movements and seasonal migrations, usually tied to the availability of food and specific breeding sites.

In some locations, the Bare-legged Swiftlet migrates to higher altitudes during the breeding season and then returns to lower elevations afterward. However, there is still much to learn about the Bare-legged Swiftlet’s migration patterns, particularly in areas where the species is threatened or has experienced significant population declines.

Studies using modern tracking technologies will help to shed light on this aspect of the species’ biology. A major factor affecting Bare-legged Swiftlet migration is habitat loss.

The destruction of natural forest habitats, in particular, has had a significant impact on the species’ population size and distribution. As a result, species movements can be disrupted, affecting breeding and overall population growth.

Conservation efforts are critical in ensuring the species’ continued survival, including habitat restoration and preservation, the control of building developments, and the prohibition of unsustainable breeding practices, among others. In conclusion, the Bare-legged Swiftlet is a highly adaptable species, capable of living in a wide range of habitats.

Although its migration patterns are not well studied, local movements and seasonal migrations are known to occur. However, habitat loss due to human activity remains a significant threat to the species’ survival, affecting both its movements and average population trends.

Therefore, conservation efforts aimed at preserving the species’ natural habitat are essential to maintain the species’ viability and conserve its contribution to the ecosystem. of the species profile, as the overall purpose is to present the information in a clear, concise manner.

Diet and Foraging

The Bare-legged Swiftlet is an insectivorous bird species that feeds primarily on flying insects, including moths, flying ants, termites, and beetles. The species is known for its high-speed aerial acrobatics, which it employs to capture these insects in mid-flight.


The Bare-legged Swiftlet has a unique feeding habit, which involves catching and consuming insects while in flight. The species can remain in flight continuously for long periods, allowing it to forage at any time of the day or night.

This behavior is facilitated by the swiftlet’s highly efficient metabolism and temperature regulation, allowing it to maintain its internal heat and energy output while in flight.


The Bare-legged Swiftlet has a wide range of food preferences, depending on the availability of insects in their respective habitats. In forested areas, the species feeds on a variety of moth species and beetles, whereas, in urban areas, it feeds primarily on flying ants and termites.

The swiftlet’s unique diet is critical to the health of its ecosystem, as it helps to control insect populations, thereby contributing to the overall biodiversity of the environment.

Metabolism and Temperature Regulation

The Bare-legged Swiftlet’s highly efficient metabolism and temperature regulation are crucial to its survival and feeding behavior. The species has a high metabolic rate and body temperature, which helps to power its constant flight and foraging activities.

These metabolic processes are supported by specialized adaptations, such as the development of a large heart and unique muscle fibers that are used in sustained flight. The species also has specific adaptations to regulate its body temperature, which is essential for efficient metabolism and energy use.

The swiftlet’s body size and structure help it to retain heat, while its feathers are uniquely designed to dissipate excess heat, regulating its internal temperature for optimal energy use.

Sounds and Vocal



The Bare-legged Swiftlet is known for its distinctive vocalizations, which consist of a series of high-pitched chirps and whistles. The species uses these sounds mainly for communication, particularly within breeding colonies and between individuals of different sexes.

Male swiftlets are known to produce distinct vocalizations during courtship and mating, which include trills and whistles. These vocalizations serve to attract females and establish territories within breeding colonies.

The species also engages in vocalizations during feeding, with individuals producing distinct sounds to communicate their location to other swiftlets. These sounds are critical to the successful functioning of the colony and the efficient use of foraging resources.

In conclusion, the Bare-legged Swiftlet is an insectivorous bird species that feeds primarily on flying insects, such as moths, ants, and beetles. Its unique diet and feeding behavior are supported by the species’ highly efficient metabolism and temperature regulation, which allows it to forage continuously and remain in flight for extended periods.

The species also exhibits distinctive vocalizations, played a critical role in communication and colony function. Understanding these behaviors and adaptations is necessary for the efficient conservation of the species in the face of anthropogenic threats.

of the species profile, as the overall purpose is to present the information in a clear, concise manner.


The Bare-legged Swiftlet is an active and social bird species, exhibiting several unique behaviors that affect its ecology and survival. These behaviors are divided into four main categories: locomotion, self-maintenance, agonistic behavior, and sexual behavior.


The Bare-legged Swiftlet is a highly mobile species, capable of sustained flight and aerial acrobatics. The species has been observed to remain in flight for extended periods, foraging for insects and building nests while in flight.

The swiftlet’s fast and agile movements make it a fascinating sight to watch for bird enthusiasts.

Self Maintenance

The Bare-legged Swiftlet engages in several behaviors to maintain its physical health and hygiene. The species bathes regularly, either by splashing water on itself or through rain or dew.

It also preens itself regularly, using its tongue to clean its feathers and manage its plumage. Agonistic


Agonistic behavior in the Bare-legged Swiftlet is primarily observed within breeding colonies, where individuals compete for nest sites and mates.

In these colonies, swiftlets engage in displays of aggression, including biting, chasing, and vocalizations, to establish their territories and defend their nests and partners. Sexual


During breeding season, Bare-legged Swiftlets engage in unique sexual behaviors, characterized by courtship displays, vocalizations, and mate selection.

Males are known to produce unique vocalizations to attract females and compete with other males within breeding colonies. They also present potential mates with saliva-filled nest cups, a behavior that serves to assess the female’s compatibility and willingness to mate.


The Bare-legged Swiftlet breeds seasonally, either on a single occasion or multiple times per year, depending on food availability and nest site availability. The species builds its nests using saliva, making unique structures that cling onto surfaces such as cave walls or building eaves.

Breeding colonies of the Bare-legged Swiftlet can be massive, with large colonies containing hundreds to thousands of nests. These colonies are also frequently home to other species of swiftlets, creating a complex ecological community.

Demography and Populations

The Bare-legged Swiftlet populations are relatively stable in some regions, but declining in others due to habitat destruction and other human activities. In certain areas, such as Malaysia or Indonesia, the species is relatively common, while in others, such as the Philippines or Thailand, populations have significantly declined.

Population declines are primarily due to habitat loss, with deforestation, land conversion, and urbanization being the main threats to the species.

Habitat degradation affects the species’ food sources and nesting sites, limiting their breeding success and population growth.

Conservation efforts aimed at protecting the Bare-legged Swiftlet’s natural habitat and promoting sustainable breeding practices are essential to reverse population declines. These efforts may include breeding programs, habitat restoration and preservation, ecotourism initiatives, and educational programs to raise awareness of the species’ ecological significance and value in Southeast Asian culture.

In conclusion, the Bare-legged Swiftlet exhibits a range of unique behaviors, including locomotion, self-maintenance, agonistic and sexual behavior. The species’ breeding behavior is also characterized by complex ecological systems and large colonies.

Although the species is relatively common in some areas of Southeast Asia, it faces population declines due to habitat loss and other human activities. Conserving the species and its habitats are thus necessary to ensure its continued survival and ecological significance in the region.

The Bare-legged Swiftlet is a fascinating bird species that exhibits a wide range of unique behaviors and adaptations that are critical to its ecology and survival. These behaviors range from foraging and breeding to vocalizations and territorial displays, contributing to overall biodiversity and ecological balance in Southeast Asia.

However, the species’ populations are threatened by habitat loss and other human activities, leading to significant population declines. Conserving the species and its habitats is thus vital to preserve its ecological significance and prevent further declines in population.

By raising awareness and promoting effective conservation efforts, we can ensure that this incredible species continues to thrive, contributing to a sustainable and healthy environment.

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