We are blessed with some wonderful chalk streams and rivers in Dorset and they are undoubtedly a prime nesting environment for kingfishers. In winter, however, kingfishers tend to move downstream nearer to the sea where, I assume, the fishing is better. The records in the Nature of Dorset database show this seasonal movement quite well with inland records from sites along the Stour and the Frome and then clusters around the outlets of these rivers into Poole harbour and Christchurch harbour. There are also sightings along the Fleet and at the mouth of the River Wey at Radipole in Weymouth; indeed kingfishers nest at Radipole.
Records for the breeding season from April to July are few and far between and this will be due the their nesting locations not being heavily watched sites together with the birds being more active at this time of year and they seem to spend little time stationary but always on the move and more likely to be missed. The bulk of the records come from August through until January when they are often at the well monitored coastal sites and at a time when they are quite happy to find a post or branch to perch on and wait for a fish to pass by rather than constantly searching for food for their young.
August and September are the prime months for sightings and one wonders whether there is actually a greater migrational movement in the autumn rather than just along their favourite river; are we seeing birds that are moving through further south during this time? I do not know whether there has been any study done on this, may be someone can enlighten me. I have never thought of the kingfisher as being migratory as such but there is no reason why it cannot be.
It is difficult to predict with any reliability where you might find a kingfisher although they do seem to stay for a while at a good location in autumn and so can be seen quite regularly there until they decide to move on. There was one for several weeks being seen daily at Holton Lee in 2018 for example. If you are looking to add kingfisher to your list I would suggest you need to keep an eye on the daily sightings during August and September to see where they are being seen most regularly.