2016-3-1 UPDATE

Can't crows see yellow?

We share our cities with many animals, among them the strange but somehow comical crows.
Recently, yellow garbage bags are being used as a countermeasure against crows, but why are they effective?
Let’s try to unravel some of the secrets of the crows’ way of life – and their eyesight.

Dreaming crow. Photo by Shoei Sugita.
Professor Sugita is also a “crow photographer,” and his dream is to publish a photo book on crows.

Playing crows
Sometimes you hear stories like these:
“There were some birds on the children’s slide in the park. The birds were actually sliding down the slide, and as soon as they got down they flew back up to the top, only to slide down again.”
“I thought I saw a bird just fall off the roof, but the moment before it was about to crash into the ground, it recovered and swept straight up again. If it had only happened once, I would have thought it had somehow forgotten to fly, but it did it again and again.”
“There were a bunch of birds sitting on a power cable, and they were swinging around and around, like gymnasts doing giant swings on the high bar.”
These birds were all crows and they were playing.

The primary aims of all animals are to stay alive and to propagate the species, and all animal behaviors tend to be explained by either of these purposes. But if you observe crows closely, you find that they do a lot of things that doesn’t seem to have anything whatsoever to do with survival or reproduction. They seem to be playing. There really is no other word for it. However, apart from crows, the only other animal that plays is man.

Known as “Professor Crow,” Shoei Sugita is frequently consulted by companies and government bodies to develop anti-crow devices.

Crow ecology
“Crows are omnivorous, and eat both meat and grains. They are extremely well adapted to their environment, they act in groups and even have a kind of society,” says Professor Shoei Sugita of the Department of Animal Science at Utsunomiya University, north of Tokyo. Nicknamed “Professor Crow,” he has written several books about these birds, but his specialty is not ornithology. His real field of expertise is neuroanatomy and the study of brain functions. However, it is precisely his approach as an anatomist, rather than as a professional ornithologist, that has brought fresh air to the area of crow studies.
“When you dissect crows and look at their brains, you find that the cerebrum is very highly developed, like in humans. The density of their neurons is also much higher than in other birds.”
Apparently, crows are capable of high-level information processing and learning. That they are very intelligent is beyond any doubt. Their digestive systems are also different than other birds. While most birds have a thick stomach wall (a gizzard), the crow stomach is like an extremely thin, soft bag. This indicates that crows developed as omnivores.
“As omnivores without any natural enemies, crows have a position outside the regular food chain of the natural world, I believe,” says Professor Sugita. They are more intelligent than other animals, they can eat anything, and don’t have any particular enemies, so they keep multiplying… rather like humans, in fact. Perhaps crows and humans have a lot of other things in common as well.

The effectiveness of yellow garbage bags
But even if people and crows have many similarities, that doesn’t mean that we always get along very well. On the contrary, various troubles are on the increase.
When Japan introduced a law prescribing transparent garbage bags, damage caused by crows scavenging through the refuse increased all over the country. The crows rip apart the garbage bags with their beaks, and cleverly pick out their favorite bits of meat from the leftovers. Apparently they are not very fond of vegetables. City crows seem to subsist entirely on human garbage, and with such an abundant supply of food, they naturally multiply. Flocks of hundreds, sometimes over a thousand crows can be scary, and their huge quantities of droppings are also a problem. Something needed to be done, and with the help of Professor Sugita, a yellow garbage bag was developed, that the crows can’t see.

A special kind of yellow that crows can't see, but the exact hue is a company secret.
A special kind of yellow that crows can’t see, but the exact hue is a company secret.

Yellow plastic bags in use in Tokyo's Suginami Ward. Crows can't see what's inside.
Yellow plastic bags in use in Tokyo’s Suginami Ward. Crows can’t see what’s inside.

“The crows attack garbage bags, because they know that they contain food. But if they can’t see the food, they will learn to fly off and search someplace else.”
Not just any yellow plastic bag will do. If it’s not a special nuance of yellow, the crows won’t be fooled.
“Actually crows have better eyesight than humans. While we can only see light in a combination of three primary colors (red, blue and green), crows’ eyes perceive combinations of four colors. The particular nuance of yellow used for the garbage bags has the effect of blocking one of those four primary colors. This rattles the crow’s eyesight, and as a result it cannot see what is inside the bag.”
In other words, the idea behind the yellow garbage bags is to use the crows’ superior vision to their disadvantage. Certain districts in Tokyo and Usuki City in southern Japan are now recommending these bags as effective countermeasures against crows.
“If they can’t find enough food, the number of crows is sure to decrease, and if the numbers dwindle, the problem of all the damage they cause around the country should also resolve itself.”
In some parts of the world, crows and ravens are associated with the “image of death,” but in Japan they are rather seen as somehow comical figures. Yatagarasu is a mythological three-legged crow that appears already in Japan’s most ancient chronicles and is now the symbol of the national football team, and the nursery rhyme Seven baby crows is familiar to all Japanese children.
If we can avoid it, we shouldn’t treat crows as our enemies. Rather, we should keep a proper distance and try to get along with them.