WASHINGTON (AP) - 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 trying to enter the country because they're discouraged by the increase 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 comparing the 69 days before Bush's mid-May announcement with 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 about 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 to about 580.

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

The reported 45 percent decline was to 166,299 arrests during the 69 days after Bush's May 15 announcement, compared with 302,447 arrests during the 69 days before the announcement.

"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."

Also Tuesday, two conservative Republicans proposed a new immigration bill that they hoped would help start negotiations between the House and Senate on immigration legislation.

The bill sponsored by Sen. Kay Bailey Hutchison, R-Texas, and Rep. Mike Pence, R-Ind., would set up privately run employment centers outside the U.S. Illegal immigrants would have to leave the U.S. and apply through the centers to return to the U.S. on work visas. Those would not operate until after the president has certified to Congress that the border is secure.