The blackcap is essentially a summer visitor to Dorset arriving in spring after a long journey from Africa where it spends the winter. Its warbling song can be heard from trees and tall hedgerow shrubs from mid April onwards into May and possibly early June but once a territory is established, a nest built and young birds have hatched and need feeding the singing stops. This is quite common species in Dorset in summer although the weekly reporting chart might not give that impression as it shows a low number of reports from week 19 to 34, that is late May through until mid-August. This is because as it is common it becomes of less interest and so is less likely to be included in a tweet.
The bulk of reports occur over the five weeks from week 14 onwards, mid-April to late May; this is when the inward migration is at its peak. After the summer lull reports from Autumn migration start coming in in mid-August and has a fairly short peak but then, rather than decline rapidly and fade away, reports keep coming right through until December. This extended autumn season is due to a 'buffering' effect. The birds that nested here leave for Africa with their young ones first but are then replaced by birds that nested much further north, often in Scandinavia and northern Europe. The bulk of these birds will then almost certainly move on further south when the harder weather hits us early in the new year and records for January and February do occur but are generally more sparse.
The distribution map shows just how widespread the blackcap is in Dorset during the summer but autumn and winter records tend to come from ringing sites and from Portland where they are seen (or trapped) on migration. The summer breeding observations come mainly, but not exclusively, from woodland sites.
To add blackcap to your list go to virtually any wooded area in May and listen for a sweet undulating burst of song. You may have to watch closely amongst the higher branches covered in leaves to actually get a glimpse of one.