What? Crows in central Tokyo have decreased by 70% over the past 20 years! Unexpected “causal relationship” with rat infestation. | FRIDAY DIGITAL

What? Crows in central Tokyo have decreased by 70% over the past 20 years! Unexpected “causal relationship” with rat infestation.

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Tokyo Metropolitan Government has reduced complaints by about 91% due to thorough garbage control measures! Northern Japan has also seen a drastic decrease due to bird flu

The number of crows in central Tokyo is decreasing.

According to an April report by the Tokyo Metropolitan Government, the total number of crows at approximately 40 locations in Tokyo decreased from 36,400 in FY01 to 8,700 in FY 2010, a decrease of approximately 76%. The number of complaints and consultations received by the TMG decreased by approximately 91% compared to FY01.

Around ’00, it was commonplace to see crows gobbling up garbage in downtown areas and shopping streets in the early morning. ……

Why have crows decreased so drastically? Naoki Tsukahara, a specially-appointed assistant professor at Utsunomiya University and president of CrowLab, who has been involved in crow research for 20 years, offers this analysis.

The Tokyo Metropolitan Government has been taking measures against crows since 2001. The Tokyo Metropolitan Government has been working on crow control since 2001. The Tokyo Metropolitan Government has put a lot of effort into garbage collection, including nighttime collection and early morning collection. I think this reduced the amount of food for crows from people’s daily lives, such as leftovers and food scraps, and downtown and residential areas became less attractive feeding grounds, resulting in a gradual decrease in the number of crows in the central Tokyo area over the past 20 years.

In addition to downtown and residential areas, crows in green areas such as Meiji Shrine have also been decreasing in Tokyo.

Crows roost at Meiji Jingu Shrine and the Imperial Palace, where they return to sleep at night and stay until morning. For crows, the closer the feeding area is to their roost, the less energy they need to expend. If the number of crows feeding in downtown areas and residential areas decreases, naturally the number of crows roosting at Meiji Jingu Shrine will also decrease.

Goshawks have inhabited Meiji Shrine for a long time, and the balance in numbers between crows and goshawks may also have something to do with it. Raptors such as the goshawk may prey on crows.

When crows were dominant, they would chase goshawks in flocks and there was no need to fear them, but as the number of roosting crows has decreased, the probability of being attacked has increased. Furthermore, it is quite possible that the increase in the number of goshawks in the city center has spurred crows to move their roosts.

As in central Tokyo, the number of crows in Sapporo seems to have dropped by half compared to 20 years ago, and highly pathogenic avian influenza has been cited as one of the reasons for the decline.

One of the reasons cited for the decline is highly pathogenic avian influenza. “Last year, there were many positive tests for avian influenza from crow carcasses, especially in the northern part of Japan.

It used to be said that crows were susceptible to avian influenza but did not die easily. They were a nuisance to farmers because they carried the virus by moving about the farm in a healthy state. In recent years, however, the number of deaths among crows with avian influenza has been increasing. The virus may have mutated and become highly toxic to crows.

As a result, there may be many crows that have died from avian influenza without our knowledge. That could be one of the reasons why the population is declining.”

A swarm of rats in downtown Tokyo…

In March, Home Rescue, an exterminator of crows, expressed this concern in a column on its website.

When crows disappear from human society, it is estimated that the number of rats and beetles that they prey on will increase.

Twenty years ago, there were approximately 36,400 crows living in Tokyo, but by FY 2010, the number had decreased to approximately 8,700.

Since around May of this year, there have been a number of postings on SNS reporting sightings of large swarms of rats in downtown Tokyo and other areas.

I am not sure. I am not an expert on rats, so I don’t know much about them, but I am sure that live animals are attractive food for crows, which love nutritious food. If rats are easy to catch, they will naturally go for them.

However, crows are not birds that actively hunt, but rather eat animal carcasses more often. Even if the number of rats is increasing, it is difficult to believe that the decrease in crows is the direct cause. It seems to me that there are other factors. For example, it could be because the amount of garbage from restaurants has increased due to the return of crowds downtown. The nighttime collection of garbage has nothing to do with rats, which are nocturnal and scavenge at night.

Indeed, if there is enough food in the downtown area, it is not surprising that rats gather there. What about crows? If garbage control measures are thoroughly implemented, are crows likely to return?

In the end, I think it depends on how you dispose of garbage.

Crows are very quick learners. They also have a very good memory. For example, if they remember eating delicious meat at a restaurant’s garbage dump, they will persistently scavenge for food. If the garbage is poorly managed, the crows will soon recognize the place as an important feeding ground.

CrowLab, of which Mr. Tsukahara is the president, has been investigating crow damage to refuse collection points in Adachi Ward, Tokyo.

Adachi Ward is making great efforts to eliminate crow damage to garbage collection points by lending out garbage collection boxes with lids and bird nets. These tools are effective, but they are inevitably vandalized when residents do not put out their garbage in a proper manner. So now, we are conducting a demonstration experiment to see what kind of garbage disposal methods are less likely to cause damage by crows, utilizing the knowledge of human behavior change and animal behavior studies.

However, the damage caused by crows is not limited to trash raiding.

In agricultural areas, ravens are a major problem. According to a survey by the Ministry of Agriculture, Forestry and Fisheries, crows are a major problem in agricultural areas, According to the Ministry of Agriculture, Forestry, and Fisheries, damage to crops nationwide in FY2009 amounted to approximately 1.3 billion yen. I have heard that some fruit farmers have closed their businesses because their fruits were eaten by crows.

Automobile manufacturers are experiencing damage to the rubber of windshield wipers and window gaskets in new motor pools and other areas, as well as damage to the bodies of their vehicles.

Power companies are having quite a problem with crow nests. Crows’ nests on utility poles can cause power outages.

Outside of central Tokyo and a few other cities, the damage caused by crows is not decreasing.

What is the trump card against crows?

In short, I think the number of crows in Japan as a whole has not changed that much.

In the end, we will have to continue to deal with crows.

Crows are part of the ecosystem, and in the first place, they live in the same sphere as humans. Unlike wild boars, deer, and other wild animals, we cannot drive them into the mountains. We have no choice but to coexist with them.

Ravens have an excellent memory, and they remember things related to food particularly well,” says Tsukahara. Some studies have shown that they can even remember people’s faces.

Even if crows are to coexist, it is important to maintain a certain distance from them.

My company offers a method to control crows’ behavior by using their voice communication as a service for countermeasures,” he says. We combine the cawing sounds that crows make when they are alarmed and play them back at the site where the damage is occurring. The crows then think that the alarmed crows are nearby and move to another location, thinking that they, too, are in danger.

Although it is difficult for crows to get used to this method of communication because they use it on a daily basis, they will always get used to it if the same voice is used repeatedly. When the crows become accustomed to the new voice, replacing it with a new one eliminates the habituation. The assumption of habituation differs greatly from previous countermeasures.

A traditional countermeasure against crows is scarecrows, but if an unfamiliar object is placed in a place they usually go, crows will be wary and will not approach it. I call this phenomenon, which crows seem to avoid, the “scarecrow effect. However, this effect is temporary. Once the crows become accustomed to it, it is no longer effective at all.

The basic principle of crow control is to reduce the amount of food that crows eat. While preventing damage symptomatically with the scarecrow effect, I think it is important to manage crow food properly, for example, by being thorough about garbage disposal and not leaving substandard crops on farmland.

Naoki Tsuk ahara, President of CrowLab, was born in Gunma Prefecture in 1979. He graduated from the Faculty of Agriculture at Utsunomiya University. In 2017, he became Representative Director of CrowLab. In 2006, he was appointed as a specially-appointed assistant professor at the Center for Bioscience Education and Research, Utsunomiya University. He is the author of “Deceiving the Crow” (NHK Publishing Shinsho).

Click here for the CrowLab website.

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  • Interview and text by Sayuri Saito

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