1. Swallows have high requirements on nesting sites and environment, and generally choose a suitable place to nest. Nesting is a big project, so swallows usually don't give up their destroyed old nests easily.
Swallows are migratory birds. When the weather turns cold, they will migrate to the south, and when the weather turns warm next year, they will fly back to the north.
3. If the old bird's nest still exists, swallows may use the old bird's nest to fix it, and then hatch eggs to feed the little swallows.
In short, although the old bird's nest may have been destroyed, the swallows may come back in a few years to repair and live in the old bird's nest.