The yellow wagtail no longer breeds in Dorset, it hasn't done for quite some time now, however it is frequently seen on passage during migration from Africa to its breeding sites further north in Britain and Europe. I say on its way to its breeding sites but actually, when you look at the weekly reporting chart you see that it is far more likely to be recorded in autumn on its way south rather than in spring going north. This is not unusual amongst passage migrant species; in spring they have one objective, to press on homeward, set up a territory and raise young. It seems that apart from dropping down for a quick snack they are not going to hang about here in Dorset when they have important work to do. The autumn is a very different scenario; they have plenty of time to make the journey south and whilst there is ample food supply here in Britain there is no incentive to move quickly on. This means that when they reach the Dorset coast they can spend a while feeding up prior to setting out across the sea. Many of these autumn birds will be youngsters who are still building up their strength and their flying skills so taking some time out is a good idea!
Yellow wagtail like to feed on flies that they find on or near cow pats and invariably they seem to be seen near herds of cows, this is not always the case but often that is where you will find them. They are usually in wild places and most often at coastal sites as the distribution map indicates. The bulk of records come from Ferrybridge, Lytchett Bay and Abbotsbury which are all good habitat for their needs but these are also amongst the best watched sites in the county and so any yellow wagtails seen on those sites are going to generate a good number of reports.
Incoming birds seem to appear between weeks 15 and 18 during May but the autumn outflux is spread over a longer period from week 34 to as late as week 40 and some stragglers are seen even beyond this. They are certainly at their peak here in late August, through September and in to October.
It is difficult to say exactly where and when is your best chance of seeing them in Dorset, the best bet is to watch the birding news and be ready to act quickly once the reports start coming in.