Holly Faulkner, Project Assistant says:

Hi Virginia, This is a good question for NestWatch. Tree Swallows use the same sized box as bluebirds (along with chickadees, wrens, and titmice), so NestWatch recommends adding a fledgling ladder to the inside of the box, just below the entrance hole, to help nestlings fledge. A fledgling ladder is simply a series of horizontal scores in the wood. That being said, we recommend that you wait to do this to your box until the young have fledged so as to reduce disturbance. Keep an eye on the box around fledging time – the nestlings will probably fledge fine, but if you have any worries, please contact your local wildlife rehabilitator before taking any action. If you’re not sure who that is, your state’s wildlife office should have a list of rehabilitators that are federally certified. If you have any additional questions, please email nestwatch at NestWatch@cornell.edu.