I've never understood the reason for a ramp at all. They fly up to nest boxes and fly up to their roosts. Most can easily hop/fly up high enough to gain entry to a pop door. You may need an exposed perch/landing spot for them at the threshold entry point, but they can do fine without a ramp.
Also, if a bird can walk up a ramp to get to the door, a varmint can too. Most varmints can't fly, so that leaves them on the ground looking up......and wishing they could fly.