Young Swallows Left Behind

 We had a late swallow nest built in our carport this summer, which produced three fledglings. The parents and one fledgling have now gone, leaving two behind. They can fly well and disappear for the day, but return each night to roost near the nest. We are concerned that these two have missed the migration and will not survive the winter. Are we right to be concerned and if so, is there anything that can be done to help them?