Online travel sites are the termites of the Internet. In the wasteland of the dot-com bust, sites like Expedia and Travelocity not only survived the bad times, they managed to thrive, turning profits during the Internet drought.

A half-decade later, though, a maturing online travel industry is facing new challenges from startups and airline suppliers. While Expedia, Travelocity, and Orbitz control 77 percent of the online travel agency market in the United States, ferocious competition is chipping away at their market share. According to a report from travel research firm PhoCusWright, released June 6, Expedia, the largest site in the U.S. and the United Kingdom, lost 5 percent market share in the first quarter, compared to the same period a year ago. Even worse, Expedia announced a 51 percent drop in first-quarter net income, which sent its shares plummeting 26 percent.

That’s not to say consumers are moving off the Internet for their vacation plans. Far from it. According to a comScore report, also released June 6, annual online travel revenues exceeded $60 billion in the U.S. in 2005, a 20 percent increase from the prior year. It’s just that consumers are slowly shifting their travel dollars away from the Big 3 sites to supplier sites run by companies like Southwest Airlines and Marriott Hotels. According to comScore, Southwest’s site landed in fifth place among the most visited travel sites in April.

Then there’s the bevy of new online travel startups that venture firms have been pumping full of cash. In the second week of June, Gusto said it raised $4 million mostly from investor William Darr for its personalized travel site, and Viator brought in $4 million from Carlyle Venture Partners and Technology Venture Partners. Other online travel startups like Kayak and Mobissimo do searches of Expedia, Travelocity, and Orbitz for consumers. Some sites concentrate on niche travel markets, like luxury travel or Caribbean travel.

Suddenly, the online travel industry is getting as crowded as Daytona during Spring Break. Or at least that’s what venture firms have decided. But that also means the dominant players will have to be that much more nimble to defend their turf. If the Expedias of the world don’t keep up, they’ll be the ones that need a vacation.