Why don all birds fly south in winter?
Of course some kinds of birds (not the species you mention) never migrate. Other kinds (not the ones you mention) migrate from the arctic to places like Colorado and states with moderate winters where there is more food for them to eat. Ducks, geese, and robins are probably staying around an area where they can find food, places where people have modified the environment which enables them to survive. You likely wouldn’t find them up in the mountains or deep the wilderness. Pigeons you see in towns or on farms are feral (domestic but reverted to a wild state). They have lost the habits of their wild ancestors from Europe.
