It happens every spring. A pair of robins makes the front page by nesting somewhere you'd never believe. OK, so we live in a mobile society, say the sociologists, but a mobile home for birds? You'd expect a barnswallow to populate your barn, and jenny to wrenovate your clothespin bag, but who'd be bird-brained enough to confiscate a skate? But wait. Consider this: put your home on wheels and if you can't get along with the neighbors—migrate.