I severely dislike the notion that there are jobs Americans don't want to do. I know plenty of people who would be ecstatic to get a government contracted bridge building job.

The problem is many of these crews are subcontractors of subcontractors and the people in the middle hire cheap labor to save on costs depriving Americans of jobs. Immigrants (both documented and undocumented) will take these jobs for a number of reasons, usually because they pay a lot more than the jobs they had in their former country.

It has nothing to do with American's desire to work and everything to do with unscrupulous companies hiring cheap labor and a labor force willing to work for below-average wages for a variety of reasons.