When I started out 'birding' back in the 1970s I was told we have resident species of birds (robins, blackbirds, etc) and migrant bird species; those that visit for the summer (swallow, house martin, etc) and those that visit for the winter (dunlin, brent goose, etc). Now I am a bit wiser! Birds need to eat and birds have wings and so birds can move to find food in bad weather; obvious isn't it? The red kite could be considered a 'resident' species as it is seen in Britain all year round but, like many other birds, when the going gets tough the red kite gets going; when the snow comes the red kite goes because it has to to survive.
In Dorset the red kite is certainly a vagrant species even though it is seen regularly but as far as I am aware, although they could well be breeding in the north of the county, they could not be considered resident. The weekly reports show that you can encounter a red kite at virtually any time of the year in Dorset. What is interesting is that there were far more reports in 2018 than in 2017 (151 v 63) and that there was a surge in reports in the May of 2018. Not only did the number of reports increase but the numbers being reported were not just individuals as is often the case but groups of birds. The winter of 2018 turned quite nasty at one point with snow across much of Britain and so the red kites gradually moved south and then in April and May they started to return to their nesting grounds in Wales and the Cotswolds passing through Dorset on their way. Is this true "migration"? Possibly not but it does show definite movement in response to changes in the weather.
The distribution map shows just how widely dispersed sightings of red kites are in Dorset and there are a good number from less watched inland sites as well as from the more closely monitored sites such as Abbotsbury, Arne, Poole Harbour and West Bexington, all of which reported good numbers.
Guaranteeing sightings of red kite in Dorset is not possible at the present time and to add it to your life list you will need to keep an eye on the news and respond accordingly!