My theory is natural selection. Birds that travel south for the winter get predated or killed by thoughtless human beings while those that travel west are more likely to survive despite our colder weather.

According to current studies our winters are actually less cold or cold for shorter periods and it is the sheer numbers of garden feeders now that make the UK a good wintering prospect. Stuart Bearhop did some work with this conclusion a few years ago and now followed up by Kate Plummer at BTO.