This figure has been especially difficult to estimate because neither country has any direct way to measure the flow of migrants illegally crossing the border.

For the United States, the collective shift means fewer migrants crossing the border illegally and possibly more debate over whether the expanded budgets for immigration enforcement still make sense.

Without question, "Operation Rio Grande" has sharply cut the number of undocumented migrants crossing through Brownsville.

The No More Deaths organization offers food, water, and medical aid to migrants crossing the desert regions of the American Southwest in an effort to reduce the increasing number of deaths along the border.

They also maintain that the United States and Canada are not prepared to accept millions of Mexican migrants crossing their borders to work in factories and farms.

Such security would be of great value in Hong Kong, which faces a perennial problem of mainland Chinese migrants crossing illegally into the territory.

These are the first migrants crossing from Asia to North America many thousands of years ago over the connecting bridge of land where the Bering Strait is now.

Using the random-sample techniques of a household survey, Dr. Bustamante estimates the flow of undocumented migrants crossing the border.

It is a repressive regime: death penalties (506 in May 2009 - of which 50% were foreigners), executions and corporal punishment, illegal detention and inhumane treatment of migrants crossing its territory to reach Europe.

The project gave disposable cameras to two groups on different sides of the United States-Mexico border: illegal migrants crossing the desert and the Minutemen volunteers trying to stop them.