Hawker dragonflies derive their collective name from the way they patrol their territory almost continuously, just occasionally resting. They defend their territory fearlessly and will even approach human beings who enter their patch to check them out! Whilst that can be a bit disconcerting the dragonfly is, of course, totally harmless to people. This hawking seems to form two purposes, the same two driving forces behind all of nature; one is food and the other reproduction. They are hunting for food and hunting for a mate! This almost constant movement can make it difficult to tell species apart.
In Dorset the most common hawker is the southern hawker but you occasionally encounter this species, the migrant hawker (Aeshna mixta). As a general rule, if the insect looks green and/or blue then it is probably the southern hawker but if it looks brown it will probably be the migrant hawker although in the east of the county a golden brown species will be the brown hawker! The migrant hawker is also somewhat smaller than the other two mentioned. To be sure you really do need to see them at rest.
Although it has the common name of migrant hawker it is a resident species which can be found near well vegetated ponds, lakes and gravel pits which is where it lays its eggs. However, numbers in late summer can be boosted by incoming migrant insects from Europe.