The Dartford warbler in Britain is right on the northern edge of the species range with it being more at home in Spain, France, Italy and along the north African coast. It is a species closely linked with heathland and here in Britain that means Dorset and the New Forest are its strongholds although it does occur even further north in Surrey, Norfolk and Staffordshire and possibly elsewhere too. Here in Dorset it is widespread on the heaths but it is not common anywhere but Dorset does provide one of the best opportunities to see them in the United Kingdom.
Unlike many of its warbler cousins the Dartford warbler does not migrate, it is a resident species and the weekly reports charts shows this with records for every week of the year with, currently at least, only week 37 being devoid if records! That said, in general there are not that many reports each week although there is a marked surge in records in April during weeks 14 to 18. This is not migration driven but occurs because it is in April, at the start of the breeding season, that the birds are most visible with males singing from the tops of gorse bushes establishing their territories. At other times of the year they are more secretive and less prominent.
There are records from 45 locations so far; these are mainly the areas of heath in east Dorset and Purbeck with the bulk of the reports coming from Arne where, thanks to the continuing work of the RSPB, they are well established and regularly seen by many people. They have also been recorded at West Bexington, Portland Bill and Durlston however whether they are resident at these sites I do not know.
A visit to the RSPB Arne reserve in spring is by far your best chance to add Dartford warbler to your Dorset list.