ZURICH -- It's geese flying south for the winter, or the swallows returning to Capistrano or the swan walk.
Moving flocks of soccer fans can tell you all you need to know.
There is an extraordinary dimension to soccer tournaments like Euro 2008. Every game, depending on the end result, brings with it a mass migration or exodus as well as a stifling influx of nationalities. They travel to one particular place for the purpose of partying, soccer and the celebration of national pride. With success comes another influx, in another place at another time. With defeat comes an exodus of biblical proportions.
Love of sport is not unknown to North American sports fans. They, too, will travel to the corners of the world to be part of a unique experience and support their nation. But that happens in relatively small numbers.
But the idea of people travelling many kilometers in numbers so great they would fill small cities, the majority of them without a ticket to a match, is a concept that's difficult to reconcile in North America.
Two nights ago when the Netherlands and Russia played, an estimated 150,000 Oranje supporters invaded Basel.
It was a relatively peaceful invasion with only 50 arrests.
The allocation of tickets for the Dutch was 6,000. Many more were in the stadium. The multitude that remained headed to the fan zone or bars. The fan zone, with three massive television screens, was a seething mass of colour and emotion.
"Even if I had tickets, I wouldn't want to go to the match," 20-year-old Dutch fan Jan Kloeder said. "I go everywhere with the Oranje and many times don't go to the match. Win and there's happiness and we come back. Lose, we leave, not happy."
Kloeder was a clone of hundreds of other young fans -- short hair, at least one tattoo, dressed in orange overalls with a banner tied to his arm that said "extreme fan."
After the game, the streets became a not-so-tranquil sea of orange. Trams were jammed to the point where transit personnel stuffed fans inside in order to close the doors.
They were rerouted to avoid the busiest areas. If you missed your stop, you went around in a circle, winding up in the same place.
This is a picture repeated time and again by fans of teams such as Croatia, Turkey, Poland and Germany.
The Dutch are going home, thanks to Russia, which surprised everyone by making the semifinal.
There was only a small gathering of Russian fans. Many didn't expect their team to be playing still. They left with expired visas. Others spent their time in embassies getting their visas extended.
"The fans expected they wouldn't need to stay past the 22nd," said Peter Kamenchenko, editor-in-chief of Russian Football Weekly in Moscow. "Now, the governments are trying to work out something quickly so visas can be renewed.
"Maybe the Austrian and Switzerland governments will just look the other way as long as the tournament is here."
Kamenchenko is a psychiatrist turned soccer writer. He understands the soccer fan.
"There is a sense of nationalism in Russia now. The fans travel to see the team. Not before. Russia is entitled to 10,000 tickets and the Russian football federation has asked for all of them. Some fans were at the 2006 World Cup even though we did not have a team there. They learned."
Russian fans rival all other fans in their zealotry.
Before most games, a giant banner is unfurled and passed hand-over-hand. It is the Russian double-headed eagle. Watching it move is spine-tingling.
It's a tradition that has taken hold in the Russian domestic league, the War of the Banners.
Club fans compete to make the biggest banner, a banner made to upset the opposition. It is unfurled and passed hand-over-hand before the game. Some banners are as big as 40-metres and cost $20,000 to make.
Before Russia's game against Sweden, a game Russia won, fans unfurled a massive banner of Peter the First, the emperor of Russia (CCT). He successfully invaded Sweden in the 18th century.
The Russian has learned the pattern. Win and the influx begins anew. Lose and the exodus begins.