I have traveled to many countries on all of the world's continents, and, always wearing my naturalist's cap, I tend to notice the wildlife, especially the birds. Some stick in one's memory, some don't, but the only country I have been where what sticks is the crows is Japan. Why is that?

Various branches of the crow family exist around the world. Some, like the raven, the carrion crow and the jungle crow, wear the familiar black raiment. Others are less obvious: the Eurasian jay with its dazzling blue wing patch, the nutcracker with its spangled plumage, and the various tropical jays of Central and South America.

The urban crow is alert for danger or opportunity.

Some are restricted to the forest, while others, more flexible in their lifestyle, are comfortable close to human habitation. A small and select group of crows, generally the black ones, are out-and-out opportunists. The Eurasian jackdaw, for example, has a fetish for shiny objects and will deftly steal silvery items to decorate its nest. Opportunists are like that. It is no good complaining about them; the only answer is not to put temptation their way.

Crows have found agriculture to their liking. Wherever there are domestic animals, there are fields with lots of droppings, and these attract insects, earthworms and the like -- all food for a hungry crow.

Many crows are happy with carrion, too, so wherever animals die or larger animals leave leftovers, they are quick to sneak in for a free treat. They will also gather where nature provides a bounty: a river dotted with dying salmon after the spawning, or the tide line after a storm has tossed up flotsam and dead fish.

All birds are motivated by simple, instinctive goals: finding food, a resting place and a place to breed. They make no value judgments.

A peregrine falcon, for example, is as at home among the high-rise buildings of New York City as along a towering sea cliff. This is not because it is a city bird at heart, but because both places supply its needs. Both skyscrapers and cliffs are high, inaccessible to predators, provide attractive ledges for nesting on, and have plenty of food nearby. For peregrines, meals often come in the form of humble rock doves or feral pigeons, which are also at home in both environments.

Crows live well off what man wastes.

Crows, too, are not attracted to cities in themselves. What draws them is the supply of good food, good nesting sites and good roosting sites. Modern cities provide them with valuable resources, particularly during winters in temperate parts of the world.

Cities are major energy consumers and major waste producers, and much of that waste is in the form of heat. Not surprisingly, birds, crows in particular, will take advantage of a winter roosting site that is a few degrees warmer than elsewhere. While commuters are returning to their suburban homes, the crows are flocking into our cities to sleep.

In other countries, the crows typically fly out of the city each morning to search for food, but not in Japan. Japan is different because of its means of garbage disposal. It often strikes me as odd that a country priding itself on cleanliness should be so messy in its handling of waste.

I am talking about the practice of piling up garbage in plastic bags out in the streets. Garbage bags are flimsy, and one doesn't need anything more than a determined finger to poke a hole in one. Any animal armed with teeth and claws, such as rats, or with a beak, like crows, must think they have died and gone to heaven. All that garbage, much of it containing edible waste, literally dumped on the streets and often at night! Talk about temptation.

In many places, citizens feel proud that they carefully cover the heap of plastic bags with a mesh net. This may stop garbage from blowing away, but it certainly cannot stop a determined jungle crow with its meat cleaver of a beak.

Repeatedly, readers have asked what can be done.

Step one: Dispose of edible waste and inedible waste separately (preferably composting the latter).

Step two: Change the method of disposal so that flimsy plastic bags of garbage do not lie out in the street. Other countries use permanent bins that are animal-proof; where they are used, one rarely sees crows foraging in the streets.

Remember, this is not a crow problem, this is a people problem. The reason crows are at an unnaturally high level here in Japan is because of the enormous volume of garbage people produce. There are so many open garbage dumps that crows benefit greatly from our wasteful way of life. Until we change our ways, crows will remain abundant.