For many bird species migration is a pretty programmed affair! By that I mean they know where they are going and when they are going. For example, swallows fly south to Africa in September, Brent geese fly south to Britain in October. For others life is a little more chaotic.
Some species move solely as a response to food supply and the impact the weathers has on their ability to feed. One of the most obvious examples of this kind of winter wandering migration in the Dorset countryside is the substantial numbers of redwing (Turdus iliacus) that come here from Scandinavia and northern Europe as the weather becomes colder there. Along with their relative, the fieldfare, they start to move as winter sets in and arrive in northern and eastern parts of Britain first but only when the going gets tough there do we start to see them in Dorset. In some winters we get huge numbers of them but if it is milder 'up north' we get less and if the weather gets too difficult for them here they fly off to France and other southern parts.
The redwing and fieldfare are species of thrush and are very much at home on farmland and can be seen in hedgerows scavenging the last remaining berries or on the ground hunting for worms and invertebrates. I have seen mixed flocks of well over a thousand birds at various locations in the county in some winters and we sometimes get small numbers in the garden too.