By Tobias Carroll As anyone who’s paid attention to the skies in the fall can attest, the migration of birds is anything but subtle. Populations of birds can go hundreds, if not thousands, of miles as they seek warmer weather, crossing vast landscapes as they go. But migration are also relatively easy to quantify — scientists have documented the routes different species of birds take, and there’s a regularity to it. At least there is most of the time. Every once in a while, though, an anomaly crops up — like, say, a pigeon traveling from the Channel Islands to northern England that ends up in …