Among Western Palaearctic birds, range expansion is more commonly observed in families with low proportions of long-distance migrants than among generally migratory families. Migrants are also disproportionately scarce in the Irish avifauna, compared with that of Britain, and scarcer among colonists of Britain than among long-established species. Colonization attempts in southeast England and in Scotland have more frequently resulted in establishment by the invader than have attempts in areas more remote from the European mainland; in several cases, these attempts have followed a history of range expansion and/or population increase in the source areas. Successful colonists lay more eggs each season, on average, than failed colonists; this difference is due, in part, to larger clutches and in part to more clutches per season. The failure of migrants to make successful invaders is attributed to their inability to compete for resources, especially habitat, in the face of competition from year-round residents. The findings otherwise agree well with predictions of the MacArthur-Wilson theory of island colonization.