If the first trip goes well, the rest of the Cubans in Costa Rica will be airlifted out. A similar arrangement may be made for about 1,000 stuck in Panama after Costa Rica shut its southern border in December. Future arrivals will get no such help, say the countries involved.

The journey of Elisabet and her fellow migrants is the latest in a series of exoduses that began with the Cuban revolution in 1959. More than 43,000 made it to the United States in the year to September 2015, a rise of 80% over the previous 12 months. The latest wave could be the last big one.

Two forces propel them. The first is Cuba’s well-known deficit of prosperity and freedom, which in some ways is getting worse. Most Cuban workers are still employed by the government, which pays an average salary of about $30 a month. Even with nearly-free housing, education and health care, “el salario no alcanza” (the pay’s not enough), Cubans are wont to say. The libreta (ration book) once gave families access to enough food, drink and tobacco; now it covers less than a quarter of their basic needs. In a lucky month a sliver of chicken or fish may appear in the bodegaswhere the ration books are accepted. In December Raúl Castro, Cuba’s president, gave a speech warning Cubans to brace for more hardship in 2016.

Cuba’s modest economic liberalisation and the unfreezing of relations with the United States since December 2014 have provided some with the wherewithal to leave. The government recently allowed citizens to sell their homes, enabling some to pay the $7,000 or so needed for tickets, bribes and transport to the United States. Others have money from American families and friends. Such high costs mean that most of the recent migrants are better off than average. An exodus of doctors prompted the government in December to reimpose a requirement that they ask for permission to leave the country.

The second push factor is the fear that, as relations improve with the United States, the American welcome to émigrés will cool. The Cuban Adjustment Act of 1966 gives Cubans a one-year route to residency in the United States. Many have taken up the offer. In 1980 125,000 Cubans joined the mass evacuation known as the Mariel boatlift, after the harbour where the voyages started. A later agreement with the Cuban government introduced the “wet foot/dry foot” policy, under which Cubans reaching American soil may stay, but those picked up at sea are sent back.

The Cubans parked on the Nicaraguan border fear that door may soon close. With reason. The American government has no plans to change the policy. But, points out Marc Rosenblum of the Migration Policy Institute, a think-tank, attitudes are hardening. Cubans who came in earlier waves were seen as refugees from tyranny. Newcomers flourishing Green Cards travel back and forth between the two countries, taking advantage of looser restrictions on travel introduced by Barack Obama. The contrast with the rejection of migrants from countries such as Honduras and El Salvador, who are fleeing appalling crime and poverty, is becoming harder to defend.

Cuba’s neighbours are slamming their doors. In December Ecuador, bowing to pressure from Cuba’s government, imposed a visa requirement on Cubans, in effect closing off the most popular route to the United States. The decision triggered rare protests by hundreds of Cubans near the Ecuadorean embassy in Havana. Costa Rica cracked down on people-smugglers in November; Nicaragua repelled border-crossers with truncheons and tear gas.

The airlift to San Salvador is supposed to clear out the last group of migrants, not encourage new ones. The $555 they will have to pay the company arranging the trip would cover the cost of plane tickets directly to the United States. But no country wants to make things easy for people passing through their territories without valid visas. Those who cannot afford the fare say they expect help. Costa Rica’s foreign ministry has said their poverty will not be an obstacle, but warns that it will send new arrivals back home.

Once the Cubans arrive at Guatemala’s border with Mexico they are on their own. They will be regarded as “illegal foreigners”, says Mexico’s National Institute of Migration, just like migrants from El Salvador and Honduras. In practice, they will be treated very differently. To reach the southern state of Chiapas Cubans cross the Suchiate river openly by bridge; those from Central America swim furtively across. The difference, say Mexican officials, is that Central American governments demand their citizens’ return, whereas Cuba does not. Another reason, no doubt, is that the United States lets the Cubans in but turns away the others. The Cubans will be given 20 days to leave Mexico, plenty of time to reach the border.

Those awaiting passage in Costa Rica doubt they are among the last. “If America changes its law,” says Generoso Machado, who was caught by Costa Rican agents after trying to cross the frontier illegally, “Cubans will go somewhere else.” At midnight on New Year’s Eve it is customary for Cubans to walk about their neighbourhoods carrying suitcases, a sign of yearning to travel in the year ahead. As long as Cuba stays poor and despotic, the tradition will thrive.