The siskins disappear in summer as do the male blackbirds and starlings and they all come back in winter. Where do they go I wonder?
Although Siskins are in the UK all round, large numbers arrive to spend the winter, so the one's you get in your garden may be European one's spending winter there. For the others it depends on where you live but a lot of birds move into urban areas in winter & go off into the countryside to breed in the summer.
EDIT : Welcome to the forum by the way, just noticed you're new on here!