Every year a small and variable number of Yellow-browed Warblers attempt to over-winter in the UK. This represents a very interesting example of bird vagrancy. Its nearest breeding range is some 5,000 kilometres away and the vast majority winter in southeast Asia. A small number, however, exhibit a phenomena known as reverse migration and head off 180 degrees in the wrong direction and end up in the UK. The current theory is that this is due to a genetic error causing them to get magnetic north and south mixed up, see here f or a much more detailed account of the causes of bird vagrancy. There is a possibility that having a small fraction of the population reverse migrate has an evolutionary advantage in that, if the habitat and climate is convivial, it would help establish a second independent population. To the best of my knowledge the Yellow-browed Warbler has never attempted to breed in the UK so these reverse migrators must either not survive our harsh w...