Redwings and fieldfares are colourful thrushes, that arrive in the UK from their northern breeding grounds for winter. They start arriving in late September to feed in fields and from berry bushes. If the weather gets very cold, they will often venture into gardens in search of food.
Fieldfare

The redwing is smaller than the fieldfare with a shorter tail. The most obvious defining feature is the rusty red patch under its wing, which is absent in the fieldfare.
It is sometimes hard to see this though, particularly in poor light or from a distance, so the other obvious distinguishing mark is the bold white stripe through the eye, and the secondary stripe below.
Fieldare

The fieldfare is a much larger thrush with a longer tail. It has a grey head with a short black stripe through the eye.
The fieldfare is sometimes mistaken for the mistle thrush. However, unlike the mistle thrush, which is a solitary bird, the fieldfare is typically spotted in groups, often alongside other thrushes, or starlings.
Alhtough a handful of fieldfares and redwings are resident in the UK all year round, mainly in northern Scotland, most birds have left the UK by April.
If you see a similar looking bird at any other time of the year it is most likely to be a song thrush or mistle thrush.