The grasshopper warbler is associated with nesting in deep reed beds but although we have some superb reedbeds here in Dorset it seems they are not to their liking and it is rare that a grasshopper warbler actually breeds here. They are seen and heard as a passage migrant and can be encountered singing from within a red bed in spring but sadly that singing does not usually represent a male in a breeding territory. They do have a more diverse range of habitat than just reed bed and they can also be found in thick scrub and dense cover, often near freshwater fens and marshes. Their 'song' is a distinctive long warbling that sounds very much like a grasshopper stridulating hence its name. Its colloquial name amongst birders is 'gropper' which you will often see used in tweets.
The spring arrivals start to happen in week 14 in April and seem to be at a peak in week 16 in late April/early May. There continues to be tweeted reports throughout May and then just a few in June. The autumn migration passage is far less pronounced and starts about week 29 in early July and goes on with occasional weekly reports until week 41 in October.
There are records from twenty four sites in Dorset and these are almost exclusively coastal as incoming birds make landfall and seek food and rest before completing their inward journey. During autumn passage birds will be seen having a final meal before setting out across the Channel on the next leg of their long journey to west Africa. Portland and Hengistbury are good sites for them at peak migration times.
Adding the 'gropper' to your Dorset list is going take some perseverance, good hearing and a considerable degree of luck!