The Federal Aviation Administration reported staffing-related delays on Monday afternoon averaging about 20 minutes at the airport in Dallas and about 40 minutes at both Newark Liberty International Airport and Austin-Bergstrom International Airport. The delays in Austin followed a brief ground stop at the airport, meaning flights were held at their originating airports until the FAA lifted the stop around 4:15 p.m. local time.

The FAA also warned of staffing issues at a facility in Jacksonville, Florida, that could cause some problems.

Just last week, U.S. Transportation Secretary Sean Duffy had predicted that travelers would start to see more flights delayed and canceled as the nation’s air traffic controllers work without pay during the shutdown, which is nearing the one-month mark.

During a weekend appearance on the Fox News program “Sunday Morning Futures,” Duffy said more controllers were calling in sick as money worries compound the stress of an already challenging job.

“And that’s a sign that the controllers are wearing thin,” Duffy said.

Earlier Monday, flights were also briefly delayed at Los Angeles International Airport, one of the busiest in the world. The disruptions emerged a day after the FAA had issued a temporary ground stop at LAX for about two hours due to a shortage of controllers. Aviation analytics firm Cirium said about 72% of the flights scheduled Sunday at LAX took off within 15 minutes of their scheduled departure times.

Most controllers are continuing to work mandatory overtime six days a week during the shutdown without pay, the National Air Traffic Controllers Association said Monday. That leaves little time for a side job unless controllers call in sick to the FAA.

Union members were expected to gather Tuesday at major airports across the U.S., including in New York City and Atlanta, to pass out leaflets to passengers detailing how the shutdown is negatively impacting the national aviation system and the workers who keep it running safely. The action coincides with controllers’ first full missing paycheck since the shutdown began.

Some U.S. airports have stepped in to provide food donations and other support for federal aviation employees working without pay, including controllers and Transportation Security Administration agents.

Before the shutdown, the FAA was already dealing with a shortage of about 3,000 air traffic controllers. Nick Daniels, president of NATCA, has said the agency had reached “the lowest staffing we’ve had in decades of only 10,800.”