Passengers hopscotching across the U.S. may book their trip on one of the major airlines such as United, but it’s often smaller, regional carriers with such names as Colgan Air that do the flying.

Regional airlines operate half the nation’s scheduled flights and are often the link between smaller communities and the national air service network.

But now, several of those carriers are being closed or are in bankruptcy court protection. They face significant challenges, as the big airlines they often fly for are phasing out smaller and costlier regional jets and cutting some low-traffic regional routes to focus on those that are more lucrative.