NEW YORK — "Strikes in July and August!" flashes the online ad, a pitch to recruit replacement nurses. "Apply now!"

When Northwest Airlines Corp. replaced its striking mechanics last week, the move drew wide attention as a potential watershed moment: Could an employer replace scores of its highly skilled workers wholesale?

But the reality — hinted at by ads like this one from Healthcare Contingency Staffing Services Inc., which supplies replacement nurses to hospitals during strikes — is that many businesses have already figured out a variety of ways to swap or substitute for employees, either to cover in a short-term crisis or permanently.

It's a reality workers will have to get used to, labor experts say.

The Northwest strike is in the spotlight because it comes during tough times for both the airline industry and for organized labor. But in many ways, it's emblematic of larger changes, reflecting U.S. employers' moves in recent years to increase their flexibility and cut costs through outsourcing, offshoring, contract labor and temporary hiring.

"It's kind of humbling," said Michael LeRoy, a professor of industrial relations at the University of Illinois who studies the use of replacement workers. "If you ask yourself, can I be replaced, lots of times you have to answer, yeah, I can be."

That trend has been reshaping the airline industry for several years, as carriers pare pieces of their business and turn them over to outside companies.

United Airlines, for example, turned over cargo handling at its larger U.S. hub airports to Swissport International Ltd. in 2003.

The move saves the carrier, a unit of bankrupt UAL Corp., about $100 million a year. The workers hired by Swissport earn about $10 an hour, roughly half what many of the unionized airline counterparts made to do the same work.

Swissport, based in Zurich, Switzerland, now provides a number of airlines with services ranging from fueling to aircraft maintenance at 51 U.S. airports.

But Northwest's carefully planned recruitment of replacement mechanics is noteworthy because the workers in question are highly skilled. That makes them more difficult to replace. In the past, specialized skills were one of workers' best means of job protection.

By week's end, though, Northwest was talking of giving permanent jobs to its replacements, and locking out the union.

Even as recently as five or six years ago, at the apex of a very tight labor market, many workers in white-collar, skilled jobs believed they were insulated from the uncertainty then enveloping workers in some relatively unskilled manufacturing businesses.

But the past few years have shown that seemingly any job — from computer programming to architecture to reading X-rays — can be done by somebody else cheaper, often overseas.

"You have lots of different ways that jobs are threatened these days," said John Budd, a professor of human resources at the University of Minnesota.

That was obvious to many unions long before the Northwest strike. When air traffic controllers walked off the job in 1981, President Reagan defied the union and sent in replacements. The government's success in facing down the strikers proved to many observers that, even when workers on the picket lines are highly skilled, they can be replaced.