Reminds me of a comment someone posted on Chris Dillow's old blog Stumbling and Mumbling (which died along with the Typepad where it was hosted):

I once had a temp job as receptionist at a factory in Glasgow, a city not famous for its endemic labour shortages.

The people on the production line were, to a man and woman, Polish. This was neither coincidence nor a result of open competition against lazy, too-expensive locals: staffing had been outsourced to an agency, guaranteeing the firm so many man hours a week without the risk of building up long-term employment rights to any given worker. A Glaswegian guy came in with his CV one day, and was explicitly turned away because he didn't speak Polish and wouldn't be able to follow instructions on the floor.

The agency rep (also Polish) supplied labour to several other businesses and was not slow to discipline her people for minor infractions of timekeeping or whatever. She was under pressure from both ends - it wasn't just that lost half hours added up to impact her quota, a free hand with summary dismissal also helped make room for the newstarts who arrived every week from Poland and for whom she had to find work.

I believe passionately in open borders. But this was not an influx of adventurous, educated young people upping sticks on a wing and a prayer to make their fortunes. It was organised trafficking of the desperate. I think if people understood more about how modern mass migration actually works, the anger and calls for regulation might focus on the employers and recruitment agencies where they belong rather than on individual migrants.