Hundreds of unionized airport service workers will convene in front of the United Airlines headquarters in Chicago, Illinois, on Thursday to demand higher wages and announce a new campaign targeting America’s largest airlines.

The protesters, who are all affiliated with the labor union UNITE HERE, work in airline catering: They’re the employees who load food onto the planes, pack the food carts and wash the dishes used in first class. Airlines subcontract that work to companies like LSG Sky Chefs and Gate Gourmet — both of which have labor agreements with UNITE HERE — but the union says companies such as United Airlines are indirectly responsible for setting wages for catering workers.

“Sky Chefs tells us, ‘Oh, we can’t give you this because the airlines are giving us this,’” said Christopher Barnes, a Sky Chefs employee based in Washington, D.C. “So we’re going to take the fight to the airlines."

Barnes, 28, said that under the current union contract he makes $8.80 an hour. Between the commute to work and the health care expenses he incurs as a result of his HIV diagnosis, he can’t afford the high rents in the D.C. metro area.

“I’m not making enough money to figure out where I’m going to lay my head when I get out of work,” he said. For a while, he was sleeping at the airport; now he relies on the hospitality of people he knows near and around Washington.

Subcontracting is a perennial concern for the labor movement, in part because of the practice’s apparent role in suppressing wages across whole industries. According to David Weil, Wage and Hour Division administrator for the U.S. Labor Department, the growth of subcontracting and similar forms of labor arrangements have played a major role in driving down compensation.

Subcontracting and using similar alternatives to directly employing workers “lead firms no longer face a wage determination problem for that work, but rather a pricing problem in selecting between companies vying for it,” said Weil in his book, The Fissured Workplace. “That change is critical because it results in fewer gains going to the workers who undertake those activities."

In other words, when large companies get to choose among competing subcontractors, they have the leverage to keep prices low. Subcontractors, working on a slimmer margin because of the pressure to bid low, end up offering lower wages to employees.

The situation of airline caterers at companies like LSG Sky Chefs and Gate Gourmet is a little different, because they, unlike the majority of subcontracted and private sector workers, are unionized. But they still face other downward pressures on their wages. When major airlines felt squeezed by the Great Recession and faced falling demand in 2008 and 2009, they responded in part by reducing their catering costs, according to the market research firm IBISWorld.

Workers affiliated with UNITE HERE say the drop off in wages for them began even earlier. Aji Tunkara, a Sky Chefs employee based in Seattle, Wash., said he’s heard from long-time employees that labor standards began to fall in the aftermath of 9/11.

“The company was helping the airlines to get back on their feet” by agreeing to reduce labor costs during a dark period for the industry, said Tunkara. “What we’re asking right now is, they’re both back on their feet, so help us get on our feet."

LSG Sky Chefs did not respond to a request for comment. In an emailed statement, United Airlines, which contracts out catering work to Gate Gourmet, said, “any questions concerning our vendors and their employees should be directed to the vendors."

Over the past few years, unions have gradually ramped up their activism around airport service work. In March 2014, the labor union SEIU successfully unionized workers at the three major airports in the New York metro area — LaGuardia, JFK and Newark.