JACUMBA HOT SPRINGS, Calif. (AP) — On many nights, hundreds of migrants squeeze through poles in a border wall or climb over on metal ladders. They gather in a buffer zone between two walls with views of the night lights of Tijuana, Mexico, waiting hours for Border Patrol agents while volunteers deliver hot coffee, instant ramen and bandages for busted knees and swollen ankles.

About an hour drive east, where the moon offers the only light, up to hundreds more navigate a boulder-strewn desert looking for always-shifting areas where migrants congregate. Groups of just a few to dozens walk dirt trails and paved roads searching for agents.

The scenes are a daily reminder that San Diego became the busiest corridor for illegal crossings in April, according to U.S. figures, the fifth region to hold that distinction in two years in a sign of how quickly migration routes are changing.

Routes were remarkably stable a short time ago. San Diego was the busiest Border Patrol sector for decades until more enforcement pushed migrants to the desert area near Tucson, Arizona, which became the top spot by 1998. The Rio Grande Valley in South Texas saw the most activity from 2013 to June 2022 as Central Americans became a greater presence.

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Migrants were arrested nearly 128,900 times on the Mexican border in April, U.S. Customs and Border Protection said Wednesday, down 6.3% from March and barely half of a record-high 250,000 in December. While still historically high, April bucked a typical spring increase.

The drop is largely due to heightened Mexican enforcement, which includes blocking migrants from boarding freight trains, according to U.S. officials. Texas Gov. Greg Abbott touts his multibillion-dollar border crackdown, while others highlight violence in the Mexican state of Tamaulipas as a deterrent on the path to the Rio Grande Valley.

Mexico pledged it won’t allow more than 4,000 illegal crossings a day to the U.S., Alicia Bárcena, Mexico’s foreign relations secretary, told reporters Tuesday. The U.S. Border Patrol arrested more than 10,000 on some days in December.

Despite the overall decline, arrests in the San Diego sector reached 37,370 in April, up 10.6% from March to replace Tucson as the busiest of nine sectors bordering Mexico. Troy Miller, CBP’s acting commissioner, said more enforcement, including with other countries, led to overall declines from March, while acknowledging “continually shifting migration patterns.”

Many migrants say San Diego is the easiest and least dangerous place to cross. They constantly check their phones for messages, social media posts and voice calls that help them plan their route and crossing.

“One hears many things on the way,” Oscar Palacios, 42, said one April morning after being driven by an agent to wait in a dirt patch where more than 100 migrants shivered near campfires. After Mexican immigration agents returned him three times to southern Mexico, the Ecuadorian man said, he gave someone he didn’t know $500 for a document that allowed him to fly to Tijuana. He then paid a smuggler to guide him to California.

San Diego’s draw lies in part because Tijuana is the largest city on the Mexican side of the border, U.S. officials say. People of nearly 100 nationalities have arrived at Tijuana’s airport this year, including 12,000 each from Colombia and Cuba, about 6,000 each from Haiti and Venezuela and thousands more from Brazil, Ecuador, Peru, China and Mauritania.

“It’s the prevalence of social media,” said Paul Beeson, whose 33-year Border Patrol career included stints as chief agent in the San Diego, Tucson and Yuma, Arizona, sectors. “There’s a lot more information out there about getting across. Air transportation has picked up and people are able to move around.”

Migration in San Diego presents a challenge because people come from a wider variety of countries — including India, Georgia, Egypt, Jamaica and Vietnam — where deportation flights can be costly and difficult to arrange, U.S. officials say. Mexicans, who are deported nearby over land, and Guatemalans and Hondurans, whose governments have long accepted frequent deportation flights, are a smaller presence there than elsewhere on the border.

The Border Patrol has been busing and flying some migrants from San Diego to other border cities for processing, a role reversal from even last year, when migrants were sent to San Diego to deal with overflow.

Migrants wait hours for agents to pick them up for processing instead of dayslong delays that were common when makeshift camps started popping up in the San Diego area about a year ago. Last month a federal judge said children in the camps were subject to custody standards guaranteeing their health and safety.

One night last week, about 70 people gathered between two walls near an upscale outlet mall. Two Honduran women were no longer able to walk after being injured while scaling the border wall; one accepted a Border Patrol ride to the hospital.

“Almost every night we have injuries from people jumping,” said Clint Carney, 58, who volunteers many nights answering migrants’ questions and serving snacks.

Near Jacumba Hot Springs, a town of less than 1,000 people, about a dozen people from Latin American countries arrived at a fork in a dirt road around 10 p.m. About 100 Chinese migrants came just before sunrise, many neatly dressed and playfully taking pictures on their phones.

Some of the Latin Americans grumbled quietly when the Chinese lined up ahead of them as Border Patrol vehicles arrived. Previously agents issued colored wristbands that were used to keep track of how long people had been waiting and who was next in line, but that practice was stopped in December.

Such staging areas have popped up in remote areas after migrants cross the border where mountainous terrain has prevented barrier construction. Mexican authorities’ increased presence in some areas pushed traffic elsewhere in the sparsely populated desert, creating new camps. One new site is a short distance from a gun club, without tents, bathrooms or other services.

San Diego shelters have been unable to house everyone who is released by the Border Patrol with notices to appear in immigration court. San Diego County Supervisor Jim Desmond said more than 143,000 migrants have been released on the streets since Sept. 13, citing Border Patrol data.

From a bus and trolley station where agents leave migrants, it is a short ride to the airport, where they can charge phones and use restrooms before boarding flights to destinations elsewhere in the U.S.

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Associated Press writer Maria Verza in Mexico City contributed.