There is one place in Dorset you are not likely to find a garden warbler and that is in your garden! The origin of some common names is certainly bewildering at times and leads to all sorts of confusion amongst people becoming interested in birds for the the first time; more than once I have had someone tell me they have had a garden warbler in their garden! The garden warbler is a bird of dense scrub and thickets and as we do not have a lot of that sort of habitat here in Dorset the garden warbler is a very restricted breeding species here and is mainly seen as a passage migrant. Like many of our warbler species it spends our winters in Africa and is with us here in Britain for just a few months in the summer.
The weekly reporting chart of tweets shows the main spring influx happening between week 16 and 20 from late April through until the end of May although a small number arrive as early as week 15 in early April and in 2019 we had one here at the end of March. The spring reports continue right through until week 24 in June and then there is a short break until the autumn departures begin in late July and early August. The autumn migration is far less pronounced and goes on until week 39 towards the end of September. That said there have been reports from week 45 and week 46 in November which seems very late.
There are records from forty five Dorset sites in the Nature of Dorset database and the distribution map reveals a high number of the sites are coastal which primarily reflects migrantional observations but there are a small number of inland locations which might be breeding sites; some, such as Fontmell Down and Kingcombe for example, would seem to be the sort of habitat they would favour.
It would seem from the sites chart that Portland in spring presents the best opportunity to add garden warbler to your Dorset list.