The buildings are there and the swallows are there, but they are “out of touch with reality”. Why are swifts long-term “residents” of Beijing?

Every spring and summer, agile figures can often be seen around many ancient buildings in China (especially in Beijing).

At first glance, they look like swallows, but their wings are narrower and sickle-shaped, and their tail feathers are not as elegantly forked as those of house swallows. Some individuals may have fallen to the ground due to exhaustion during migration, giving us the opportunity to observe them up close: their beaks are extremely short, their mouths are large, and their toes have an unusual front four pattern.

This type of bird has a nice name: Beijing Swift (Apus apus pekinensis).

Comparison of the common swift (left) and the house martin (right) [not actual size comparison]

Image source: homeandroost

Swifts are not swallows

The swift family includes about 96 species of birds. Their sickle-shaped wings give them super high flying speed and excellent agility, allowing them to easily catch flying insects. Rather than saying that swifts look like swallows, it is better to say that the swallows' family, Passeriformes, is the plagiarist, because the history of the swift family is much earlier than that of the swallow family, and can be traced back to a very distant era.

Swifts belong to the family Apodidae, which is a large family of nocturnal birds. The closest relatives are the Trochilidae, which are specialized in sucking nectar and live like bees, and the Hemiprocnidae, which are like swifts with feathers on their heads and have more colorful feathers.

Unlike swifts, crested swifts prefer to build nests on branches. Source: YouTube

The entire family of night birds always gives people the feeling that they are obviously birds in appearance, but their behavior is not bird-like at all! Not only have they independently evolved echolocation similar to bats and dolphins among mammals many times, but they can also reduce their metabolic level and enter a "low-consumption" dormant state like some amphibians and reptiles to survive the season of food shortage.

Phylogenetic tree of nocturnal birds Source: References

At the same time, nocturnal birds are also a group with a long history. The earliest reliable fossil records can be traced back to the Paleocene, when the dinosaurs were still alive. In the subsequent Eocene period, the diversity of nocturnal birds was already quite high, and fossil records can be found for most living groups.

The earliest known swift, Eocypselus, lived in the Green River Formation of North America 52 million years ago. Detailed studies of its fossils show that its morphology is quite similar to that of modern swifts, except that its beak is thicker and its toes are slender and suitable for grasping tree trunks, more similar to modern crested swifts. This suggests that early swifts may have had a similar arboreal ecological niche, rather than being active flyers.

Tomography (left) and reconstruction (right) of the fossil of the proto-swift

Image source: Made by the author

During the same period, the Scanian swift Scaniacypselus in Europe also showed the same characteristics. Before the Miocene, the fossil record of the passerine order to which the swallow belongs was extremely scarce. Although there is no conclusive fossil evidence, it is certain that the swallow evolved into a form similar to today in a very recent geological history period.

In short, the similar appearance between swifts and swallows is just the result of convergent evolution for "aerial insect hunting". They are not related at all! In fact, swallows are more closely related to penguins than to swifts! In fact, it was not until 1777 that scientists noticed that swifts and swallows were very different, and swifts were separated from the genus Hirundo and established in the genus Apus.

Born to fly, "out of touch with the earth"

The movie Days of Being Wild mentioned a bird that almost fell to the ground, and once it fell to the ground, it would mean death. Although the movie is certainly fictional, if we have to find a bird that is closest to it in reality, it would be the swift.

In fact, apart from the two-month breeding season when they need to stay in the nest overnight, Beijing swifts rarely land for more than two hours in the remaining nearly ten months, and spend more than 99% of their time in the air. It should be noted that swifts cannot fly back to the sky after landing, and can only sit and wait for death to come. Although it has been confirmed to be a rumor, it is undeniable that if it were not for breeding, swifts would almost never consider landing in their entire lives, literally "out of touch with the ground."

Swift: "Is there anything that can't be solved in the air?" Image source: See watermark

Swifts are light and have large wings, with extremely low wing loads, which can make full use of various updrafts to save energy. Thanks to this extremely abnormal body structure, swifts can fly as easily as breathing. Swifts can hunt while flying, clean themselves while flying, drink water while flying over the water, and even mate in the air.

Swifts' unique way of drinking water Source: birdGuides

In addition, swifts emit high-frequency calls as a positioning aid when flying, especially for the swiftlet Aerodramus [the producer of bird's nests] that nest in deep caves, echolocation ability is also used to explore the way. However, unlike bats, the echolocation of swifts can only explore the existence of objects, not images.

As for the very important issue of sleeping, swifts solve it in a similar way to mammals such as whales and dolphins, using slow wave sleep (SWS). In short, one side of the body is awake while the other side is asleep with eyes closed. The limb movements of birds are controlled by the brain stem, which means that even if the brain falls asleep, the wings can still flap.

In order to get more rest, swifts will also use rapid eye movement (REM) sleep when necessary, which is regular sleep. At this time, for safety reasons, swifts will take advantage of the rising air currents to soar to an altitude of nearly 3,000 meters, and use this rising time to catch up on sleep.

The building is here, the swallows are here

In 1870, the ninth year of the Tongzhi reign of the same era in China, British naturalist Robert Swinhoe described and named a new subspecies of the common swift, the Beijing swift, based on the specimen collection site in Beijing. Compared with the nominate subspecies that breeds in Europe and has dark feathers, the Beijing swift's feathers are much lighter, tending to be brown. Because it often appears in a series of ancient buildings such as pavilions, towers, and pavilions, the Beijing swift is also known as the "building swallow" in the folk.

Beijing Swifts Photo source: Gobal Times

Unlike their relatives living in Europe, the breeding grounds of Beijing swifts are basically concentrated in Beijing, and most of them gather in the urban area of ​​Beijing, while the swift population near the suburbs is relatively small. As long-term residents of Beijing, Beijing swifts stay here for about 120 days every year. They will rest, eat, and complete the important mission of reproducing offspring in this ancient city of Beijing.

Swifts have extremely specialized feet to adapt to flying. Not only are they so short that they are called "birds without feet" [which is also the meaning of the genus name of swifts], but their four toes point forward and their feet are wide, which is completely unlike a pair of bird feet. They have evolved similarly to bats and are suitable for grasping vertical rock walls. This makes it difficult for them to build nests using regular trees like other birds, and the mortise and tenon joints used in ancient wooden structures seem to be naturally prepared for swifts with "inconvenient legs and feet"!

The little feet of the Beijing Swift. Source: Wiki

Since Emperor Zhu Di of the Ming Dynasty moved the capital to Beijing in 1421, large-scale construction activities in the ancient city of Beijing have continued for nearly four hundred years, and only gradually stopped in the late Qing Dynasty. Such a large-scale ancient building complex naturally attracted a large group of swifts to settle here.

The ancients realized very early that swifts prefer to use artificial buildings to build nests and reproduce, and to a certain extent, they prospered and declined together with the ancients' towns. So much so that when Sima Guang recorded Tuoba Tao's invasion of the south in "Zizhi Tongjian", he used the phrase "the spring swallows returning home could not find houses and were forced to build nests in trees" to describe the tragic situation of the massive population loss after the war! The original text recorded: "In the counties and prefectures passed, there was nothing left, and the spring swallows returned and nested in the trees."

The fate of Beijing swifts is closely linked to the buildings and temples of Beijing. In the 1950s and 1960s, a considerable number of ancient buildings were demolished to develop transportation. This was undoubtedly a disaster for the swifts, who love their homes very much, and directly led to a population decline of about 90% compared with the early part of the last century.

In the 1980s, for the purpose of maintaining ancient buildings, bird-proof nets were laid out on a large scale to prevent the swifts' feces from polluting and corroding wooden structures, which further compressed the swifts' limited living space.

Fortunately, the precarious situation of Beijing's swifts has finally attracted people's attention! In order to solve the housing pressure, the Beijing Olympic Forest Park built a Swift Tower in 2008. Unfortunately, due to the unreasonable design at the beginning, it did not attract swifts, but welcomed flocks of sparrows. Ironically, this can be regarded as "an unintentional success".

In 2018, a project titled "Ancient Building Protection and Urban Ecology Research - Taking the Beijing Swifts Living on Zhengyang Gate as an Example" was officially launched. The study confirmed that swift droppings do not have a substantial impact on ancient buildings. The bird-proof nets were removed one after another, and cameras were specially installed in some important breeding areas to facilitate investigation and research at all times.

Image source of rescued swifts released into the wild: https://birdingbeijing.com/

At the same time, swifts are also adapting to the new environment. They have begun to build nests on some modern buildings, such as overpasses. Surveys and studies in the past two years have shown that the population of swifts in Beijing has increased to more than 10,000. I believe that the number of swifts in Beijing will increase. I believe that Beijing, an ancient city with a long history and openness and tolerance, can allow the drifting swifts to live in peace and prosperity for a long time.

References:

[1] A 30,000-km journey by Apus apus pekinensis tracks arid lands between northern China and south-western Africa

[2] Sadanandan KR, Ko MC, Low GW, Gahr M, Edwards SV, Hiller M, Sackton TB, Rheindt FE, Sin SYW, Baldwin MW (2023). Convergence in hearing-related genes between echolocating birds and mammals. Proc.Natl.Acad.Sci.USA120(43):e2307340120.

[3] Ophthalmology of Strisores: Nightjars, Frogmouths, Swifts, Hummingbirds, and Relatives

[4] Hedenström, A., Norevik, G., Warfvinge, K., Andersson, A., Bäckman, J., & Åkesson, S. (2016). Annual 10-month aerial life phase in the common swift Apus apus. Current Biology, 26(22), 3066-3070. Morganti, M., Rubolini, D., Åkesson, S., Bermejo, A., de la Puente, J., Lardelli, R., ... & Ambrosini, R. (2018). Effect of light-level geolocators on apparent survival of two highly aerial swift species. Journal of Avian Biology, 49(1), jav-01521.

[5] Science Popularization in China - The legendary "bird without feet" turns out to be right next to us!

Author: Xie Honghan, popular science author

Editor: Guru