The arrival patterns of summer visitors to Britain and Ireland were monitored from 2002 to 2004 using large numbers of birdwatching lists collected by Migration Watch, an Internet-based survey. Records were only accepted from registered observers, and procedures for data validation were implemented. We show here how data on the frequency of occurrence from birdwatching lists can be analysed to estimate the timing and duration of the migration period. Aerial insectivores showed clear species-specific arrival patterns, with sand martins arriving first, followed in succession by swallows, house martins and finally swifts. Wheatears showed two peaks of arrivals, one for the British population and one for passage migrants from Greenland. The progression of arrivals from south to north and variation in arrival timing between years were also demonstrated. The method offers considerable potential for studying migration phenology at large spatial scales, and within Britain and Ireland it is now being applied throughout the year within the BTO/RSPB/BWI BirdTrack project. It could potentially be implemented at a continental scale, at which it would provide an important tool for measuring the growing impacts of global climate change on bird populations.