There are many birds called "swallows" in China, and they mainly come from two groups - the swallow family and the swift family. The genetic relationship between these two groups is actually very distant, but their appearance and flight characteristics are very similar, making it difficult for ordinary people to distinguish each other.
△ There are obvious differences in the feet of Swiftidae and Yanidae
This is the result of convergent evolution, because both are highly specialized aerial predators that are experts at catching those small flying insects in flight.
In terms of flight ability, the Swiftidae are more powerful than the Swiftidae. The Swiftidae spend most of their lives flying and are even known for being "unable to survive landing" (this is not true), while the Swiftidae are slightly inferior and often rest on telephone poles.
Because of their predatory behavior of preying on flying insects, most of the swallows in China (except for those species that live in places such as Yunnan where the seasons are not obvious), and almost all of the swifts are migratory birds, and they will trace their food resources and conduct seasonal migrations;
At the same time, because both have powerful flight capabilities, their migration distance is much longer than that of other migratory birds (the migration distance of migratory birds is closely related to their flight capabilities).
△ Swift
△ Jiayan
We mentioned earlier that the flying ability of the Swift family is slightly worse than that of the Swift family, so the migration distance of the Swift family is also shorter.
Some species of swallows in China do not actually fly out of the country. They start migrating southward in July and August, eating and drinking all the way to Hainan, Yunnan and other places to overwinter and then return;
There are also some that migrate farther away and will overwinter in Southeast Asian countries, and even further ones will go to the Southern Hemisphere - they will cross the ocean and spend the winter in Australia.
As for the Swift family, they rely on their extremely strong flight capabilities to migrate very far distances. Many swifts overwinter south of the equator, mainly including southern Africa, Australia and other places.
However, at least some of them do not go all the way south, such as the Beijing Swift.
The Beijing Swift is actually a subspecies of the Common Swift (Apus apus pekinensis). This species currently has 4 subspecies described and is distributed in most areas of Eurasia.
From 2014 to 2018, a Chinese scientific research team tracked the migration of Beijing swifts [1].
During this period, researchers equipped a total of 66 migrating Beijing swifts with micro-photosensitive locators, and data from 25 samples were recovered.
These recovered data show that the Beijing Swift departs from Beijing at the end of July and early August and first enters Mongolia in the northwest direction - these areas also have other common Swift subspecies, which can be regarded as a convergence.
Then, they entered northern Xinjiang from Mongolia, and then crossed the Junggar Basin into the Middle East. This journey totaled several thousand kilometers, but they only took about half a month to complete.
In mid-August, Beijing Swifts crossed the Red Sea and entered Africa, then headed south, finally reaching their overwintering destination - South Africa in early November. They flew 14,733 kilometers and took an average of 111 days.
Of course, they don't fly all the time. They also eat, drink and migrate along the way, stopping briefly in many areas.
During the entire winter migration, the number of long-term stays of Beijing swifts ranged from 2 to 6 times, with an average stay time of 71 days, accounting for about 64% of the total migration time.
The areas where they stay mainly include around the Congo Basin, as well as the southwestern coast of the Red Sea and the southern coast of the Caspian Sea, and these areas are just able to provide them with short-term food resources.
On the other hand, the return migration cycle is much shorter - only 64 days on average, and the number of stopovers is also much less, with the maximum stopover being only 2 times, and the longest stay being 36 days.
Compared with where the "swallows" migrate and overwinter, why do these animals go through so much trouble to migrate? It is more interesting.
Why do swallows migrate?
I wonder if you have ever thought about it, Beijing Swifts live in both Beijing and South Africa, so where do they originate?
There is no generally accepted answer to this question, but it is more likely that they originated in Africa.
Not only Beijing swifts, but other swallows, and even other migratory birds that migrate over long distances, may have originated near the equator where the weather is warm and the seasonal changes are not obvious [2].
As for the north, where the seasons change obviously, that is just the breeding place they choose, so you will find that the "swallows" build nests as soon as they come back.
△ Barn Swallow’s Nest
Knowing this, the important reasons why migratory birds migrate become obvious.
One hypothesis is that by breeding away from where they live, reproductive success may be higher.
Migratory birds that migrate have their genes preserved due to increased reproductive success, and as time goes by, their migration distances become more and more distant, even across the globe.
This hypothesis is not just a random thought; it is based on the fact that migrating birds raise more offspring than their close relatives who stay in the tropics.
Normally, migratory birds can raise an average of 4-6 chicks, while their close relatives who stay in the tropics can only raise 2-3 chicks [2].
This reproductive success is determined by many factors, including seasonal insect outbreaks in the "northern" that provide greater food resources, migration that reduces potential predators, and many other factors.