The great northern diver breeds in the Arctic and migrates south for the winter. Most birds seen in Britain come from Iceland although some come from Greenland and even as far away as Canada. In winter they are a frequent sight around the northern shores of Britain and Ireland and a small number venture as far south as the Dorset coast where they are seen in variable numbers each year. They are not numerous here but, as notable birds, those that are here generate a good number of reports from local birders.
They start to appear in favoured Dorset locations around week 46 which is early November and they remain until the following May although reports seem to start to diminish from week 5 at the beginning of February. A couple of odd reports have been seen from as late as June but this is certainly not the norm and are probably non-breeding birds but even they will head north at some point and you would not expect to see one during July, August or September with the chance of an early returning bird in October before the main influx in November.
There were almost twice as many reports in 2018 as there were in 2017 although that may not mean twice as many birds present.
Although birds of the open sea they demonstrate a distinct preference for the sheltered waters of Poole and Portland harbours for their stay here with just occasional reports from more exposed and open offshore locations. Offshore reports are often of birds on the move whereas the harbour birds seem to be settled and feeding here.
The most likely sure sighting of a great northern diver is Portland Harbour in winter but they can also be seen close to Poole town at Baiter and even on the boating lake at Poole Park.