The number of illegal immigrants caught trying to sneak into the United States has dropped since President Bush ordered the military to help tighten the border, the head of the Border Patrol said Tuesday.

Officials surmise that part of the reason is that fewer people are even attempting to enter the states because they're discouraged by the ramp-up in efforts against them.

Immigrant rights advocates think the migrants may just be shifting entry points, crossing at more remote and dangerous areas.

Whichever it is, Border Patrol chief David V. Aguilar reported a 45 percent decline in the number of people arrested along the U.S.-Mexican border, when considering the 69 days before Bush's mid-May announcement to the 69 days after.

That's a much greater decline than normally seen in the summer months when southern temperatures rise dangerously and discourage some people from making the trip, officials said. The seasonal decrease was 27 percent last year and 29 percent in 2004, said Customs agency spokesman Michael Friel.

Aguilar spoke at a press conference with Lt. Gen. H. Steven Blum, chief of the National Guard, which is sending some 6,000 troops to help with logistics, communications and other duties and thus free up border agents to do more enforcement work.

"We are becoming more efficient," Aguilar said of the operation since troops began arriving.

Some 4,500 National Guardsmen are in place in California, New Mexico, Texas and Arizona, with the rest due Aug. 1, said Blum.

Their arrival so far has freed 250 border agents from support duties, a number expected to grow ultimately to about 580.

"It's positive, it's real," Aguilar said of the effects of the effort.

The reported 45 percent decline was to 166,299 arrests from May 16 to July 23 this year, compared with 302,447 arrests in the same period last year, Friel said.

"We have more eyes and ears on the border, more agents and apprehensions are down," he said. "I think it's logical to say that we are gaining control of that piece of the border," Friel said. "Something's going on."