Autumn is a time of migration, especially in relation to birds. Many have left to warmer shores. Of course it's usually not the case that they can't stand the cold but that the changing seasons have taken away their food source. In most cases that means insects that were used to feed adult birds and raise the young.
The other side of the coin is that we are in receipt of new birds - we principally think of wildfowl and Thrushes. They have different diets that are still available.
Another group that join us comprises wading birds that arrive from the Arctic. Birds such as Knot, Purple Sandpiper and Sanderling (pictured). As winter progress they turn whiter to blend in with our winter landscapes. Some Sanderling winter much further south, indeed we have seen them at the Equator on our first Galapagos adventure.