You may not have expected that the "little swallow" who arrives late and leaves early is undergoing the test of climate change丨The earth is calling

Editor's note: Regardless of your position on the causes of climate change, no one can deny that it is happening. In addition to the seemingly distant signs of climate change, such as melting sea ice in the Arctic Ocean and intensifying drought in East Africa, there are already plenty of signs around us that are demonstrating the power of this global shift.

Yes, climate change is not just about warming and extreme weather, it also has many subtle manifestations. In the coming period, I will work with you to discover these climate change stories that are all around us. Stay tuned~

Hirundo rustica, a natural spirit familiar to Chinese, this small migratory bird likes to nest in people's habitats and is good at preying on pests, and has always been a beneficial bird that has attracted much attention and attention. Because the time of migration and return is highly consistent with agricultural production activities, and because the house swallow living around people is easy to observe, we have always had the practice of observing the return of house swallow to guide agricultural production.

However, in recent years, house swallows in many regions have shown a trend of "arriving late and leaving early" – they return later and migrate south earlier. Behind this phenomenon, in fact, it is also inseparable from the impact of climate change.

Jiayan's "late arrival and early departure"

The continent is a large agricultural producer, so the recording of natural phenomena that can guide agricultural production continues to this day, thus forming a set of special "calendars" that mark seasonal changes with the behavior of animals and plants in nature—phenological calendars. As an organism that is very sensitive to seasonal changes and easy to observe, the house swallow has always been the focus of the phenological calendar in the northern part of the mainland. According to the requirements of the "Agricultural Meteorological Observation Specification", the observation of the house swallow in various places is mainly focused on the first and extinct periods, the former is the date of the first time the house swallow was seen in this area, and the latter is the date of the last time the house swallow was seen in this year, and the interval between the two is the meaning of how many days the house swallow stayed in this area this year.

Since the interval between the first sights, the first sights, and the first sights vary in each region, I will take my hometown of Qingdao, Shandong Province as an example. In previous years, the house swallow in Qingdao generally began to appear in April/May, the average date was around April 19, and the southward flight was extinct in September/October, and the average date was around October 2, with an interval of about 166 days. However, studies in recent years have found that there is a clear trend of delaying the first sighting period, advancing the extinction period, and shortening the interval in Qingdao, and the fluctuations in some years are even very drastic, such as in 2017 and 2018, when the Jiayan left Qingdao on September 6, nearly a month earlier than in normal years.

This year, my own observation also found that around September 13, there were almost no house swallows in Qingdao (some could still be seen in the villages near Rizhao City in the south until September 22). In other words, the total time that Qingdao citizens can see house swallows every year has decreased by 1/5~1/6, which is not insignificant.

What is the reason for the "late arrival and early departure" of the house swallow? It is generally believed that it is the impact of climate change.

If you take a closer look, isn't there something wrong? Isn't it global warming? Since it's warming, can't the swallows stay a little longer? Why did they go earlier?

This is why I do not recommend that we simply equate climate change with "global warming", climate change is far more complex than "warming", on the whole, there is indeed a general trend of increasing greenhouse gases and overall rising temperatures, but in specific time periods and specific regions, it is more reflected in rainfall, cloud cover, wind speed, sunshine hours and many other levels of disorder, and many of these factors may have a more significant impact on animals than simple temperature changes.

In the case of house swallows in Qingdao, for example, the change in sunshine hours may have played a more decisive role.

Sunshine hours, misplaced "alarm clocks"

Sunshine hours are the amount of time the sun's center reaches the ground from the eastern horizon to the western horizon without any shade. In recent decades, the increase in cloud cover in various parts of China has blocked more direct sunlight from the ground, so there has been a nationwide trend of shrinking sunshine hours, especially in the Huanghuai region, which has seen a sharp reduction in sunshine hours since 2000, with a reduction rate of about 9 hours per year, in North China, Jianghan, Jianghuai and South China at a rate of 7 hours per year, and in the southwest, Jiangnan and Northeast China, the rate of reduction is about 3 hours per year. In general, the coastal areas have the fastest reduction in sunshine hours, and the most obvious decrease in sunshine hours in summer, and Qingdao is one of the most significant areas in mainland China.

Light stimuli are the most important indicator of the change of seasons for most animals. In the middle and high latitudes, the length of day and night varies depending on the seasons, and if we assume that a region has 230 hours in September and 220 hours in October, then migratory birds like the house swallow will feel the natural shortening of daylight hours and know that it is time to go.

Now, with the shortening of daylight hours due to climate change, the daylight hours in September have become as short as October, and migratory birds have the illusion that "it's time to go", urging them to leave earlier, which has also led to their "early departure". The principle of "lateness" is basically the same, and the migratory birds returning to the north may also feel that "the sunshine hours on the road are not enough, and my northern hometown should not yet be in spring", delaying the pace of going north.

Of course, the number of hours of sunshine is only one of the reasons for "late arrival and early departure", and another important reason for their "early departure" may also be related to the misalignment of food peaks.

Feast on insects, but start early

From the moment they return to their habitat in the northern hemisphere, they enter a "race to the death". The old swallow has to find its own old nest to repair and renovate, and the new swallow has to find its own nesting place to build a nest with mud, and then it is courtship, mating, laying eggs, and bringing babies. Newly born nestlings are racing against time, counting from the day they break out of the shell, the nests can already fly from high places on the 19th day and back to the nest from the ground on the 21st day. Because the time to stay here is only a hundred days, and the chicks only have so much time to grow and mature to complete their first long-distance migration, and autumn is coming in the blink of an eye.

Behind this series of work, there is a big thing that is inseparable -- eating! Yan takes food as the sky, and there is a sufficient supply of food, so that the large amount of work behind can be completed beautifully. The swallows that have just migrated back to the north are physically exhausted, so they have to eat a good meal before they have the physical strength to give birth to a healthy baby. Newborn babies have a big appetite and can grow fast when they are full. Moving south is a long journey of thousands of kilometers, how can you have the strength to fly if you don't have a good store of nutrients?

Obviously, the availability of food in the breeding grounds is of high quality and will determine whether everything can be done according to the schedule.

However, the increase of accumulated temperature in winter, the early warming in spring, and the change of rainfall caused by climate change have a significant impact on the early wintering and recovery of insects, and also lead to the earlier migration of migratory insects to higher latitudes. Originally, when the house swallow returned to a certain place to enter the breeding peak, it happened to overlap with the outbreak period of insects, and the house swallow could use the abundant food resources to feed the chicks, but now, the house swallow has not returned, the insect has entered the outbreak period, and the migratory insects have begun to spread northward, although the longer temperature suitable period can allow a variety of insects to reproduce several more generations in a year, but these new breeding peaks often occur on their way to the outward (mainly to high latitudes), and the breeding house swallows that need rich food have time and geographical dislocation, resulting in a decrease in the reproductive success rate of the house swallowThe growth of young chicks is slower, which affects the survival rate of young chicks during their migration to the south, or makes it difficult for domestic swallows to obtain sufficient food more efficiently in late summer and early autumn, and is forced to fly south in advance in search of food.

For example, there is an important agricultural pest - Oriental armyworm, they were originally wintering in the area of 27 °N ~ 33 °N in the mainland, the northernmost limit of wintering is probably in the Qinling Huai River line, in spring, when the armyworm living here awakens and develops into an adult and begins to mate, the house swallow that migrates here also happens to come back; after that, the armyworm migrates northward, 2~3 generations later to the northeast of the mainland, and the northeast house swallow also comes back during this period, and it happens to overlap. A large number of outbreaks of adult insects fly and mate, which brings sufficient food sources for the migrating house swallows, which is conducive to the rapid replenishment of physical energy and the mating period. But now, climate change has caused the overwintering area of armyworms to expand northward by 1 latitude, and the warm winter and early spring have made the recovery of armyworms earlier, and the corresponding time to develop into adults and mate has also advanced. In addition, the armyworm also spread further northward, and they will also migrate back to the southern wintering grounds earlier, which also affects the time point of nutrient storage in late autumn.

Let's look at another type of insect – the cicada. Cicadas and house swallows are the same phenological emblematic creatures, the beginning of the cicada song and the end of the cicada song in the period of the year are also more detailed, in recent years, this year's observation found that the earlier spring rain is conducive to the earlier emergence of the cicada, the summer sunshine hours and wind speed changes are conducive to the mating of cicada adults, but the cicada this insect died after mating and laying eggs, and on the whole, the cicada in the annual fading (final song period) is also advanced.

It is important to note that cicadas and armyworms are not the only food for house swallows, and may not even be the main food for house swallows in many regions, and their activity patterns are far more complex than those we have just mentioned. This story is just a microcosm of the impact of climate change on many agricultural pests, and I will write about it later when I have the opportunity to dedicate it to this topic.

The old swallow returns to the nest, and there is no nest to return to

When it comes to the habits of house swallows, people are most familiar with the fact that they like to build nests under people's roofs, and every year the old swallows will return to the nest. This is a manifestation of the co-evolution of the house swallow and humans, and the whole genome study found that about 7,700 years ago, the house swallow gradually changed from simply using natural caves and rock cracks to build nests, and gradually changed to adapt to human buildings. The probability of direct impact is also lower, and there is usually a large amount of agricultural land or grazing areas around early human settlements, and insects are more concentrated, which is more conducive to the life of house swallows in terms of safety and convenience. Existing studies have also found that the global population of house swallows has increased rapidly after using artificial structures as nesting environments, and even in North America and Canada, where the population of house swallows has declined most severely, the population is still larger than 500 years ago (the era of rapid increase in European and American settlements).

However, in the last 100 years or so, global urbanization has accelerated, the size of rural areas has been greatly reduced, and production patterns have also changed, such as grazing to intensive breeding, and the cultivation industry relies on agricultural insecticide, which has reduced the advantage of "living near humans and finding food".

The transformation of the building structure also affected the nesting of house swallows, and the wooden beam rafters that were originally the most suitable for nesting were replaced by cement and tile walls, and this kind of structure is very smooth, the mud is not easy to stick, and the difficulty of nesting increases, forcing the house swallow to build its nest only on nails, lamps, wires, etc. on cement buildings. In rural areas, semi-open barns and cattle sheds are becoming less and less, and indoor doors that were originally open for daylight and ventilation have also been changed to normally closed due to the spread of glass windows, screen windows, and range hoods. In the city, in the process of renovating old communities, unit doors are gradually installed, glass windows in the corridors are closed, and the façade decoration of shops on the street has increased the obstacles to the use of artificial buildings to build nests.

A bird's nest attached to an electric light

As we just said, urban buildings are not friendly to nesting of house swallows, and in the context of population urbanization, most of the population is moving to the city, which leads to the fact that for most people, the frequency of daily sighting of swallows has decreased, and it is almost impossible to see nesting swallows at home in the city.

But "fewer swallows" is not just an illusion, the size of the global house swallow population is shrinking, and the causes of this result are more complex, but it is also inseparable from the effects of climate change, and we will continue to discuss this in the next issue.