Many species of North American landbird have been recorded in Great Britain as vagrants. Most occur in autumn; southwest England attracts the greatest proportion, but northern and western Scotland comes a close second. Occasionally birds overwinter, and some species (e.g. Baltimore oriole) are more prone to this than others. Vagrancy also occurs in spring, and some species (e.g. white-throated sparrow and dark-eyed junco) do in fact have more records at this time than in autumn. Weather systems are thought to be the primary reason for the occurrence of birds in autumn; some birds seen in spring may simply be overshoots, although ship-assistance may also play a part.