MANNS HARBOR, N.C. (AP) - This travel advisory is for the birds.

North Carolina's Transportation Department is reminding drivers of a lower speed limit on an Outer Banks bridge to protect purple martins, which roost annually on the span over the Croatan Sound.

The speed limit on the William B. Umstead Bridge in Manns Harbor is 20 mph at dusk and dawn to protect the more than 100,000 purple martins that roost on the west end of the bridge as they prepare for their annual migration to Brazil.

The birds roost under the bridge at night. They leave at dawn to feed and return sunset. The flock is so large during its peak that it can be seen on Doppler radar.

The department works with the Coastal Carolina Purple Martin Society to protect the birds.