A covert feather is one of a set of feathers which cover other feathers. The coverts help to smooth airflow over the wings and tail and protect important flight feathers. You can see them inside the red outline on the photo of a Northern Mockingbird that was banded during our Fall Migration Station.
Since most songbirds partially molt their covert feathers during their first molt, these feathers can be used to help banders determine the age of a bird, by examining the difference between the newer fresher adult feathers and the older worn juvenile feathers.
This interesting post was originally seen on a Facebook post from USGS Eastern Ecological Science Center.