If she was not sitting on eggs before (is the nest empty?), she might be off the nest to mate, eat, drink, bathe, gather more nesting materials, or could have been killed by a predator. Bird nesting behavior is outside of the realm of our expertise, but you can inquire with wildlife biologists at the MD Department of Natural Resources. An email can be sent to <personal data hidden> (especially if you want to share photos) or, if you would like to speak to someone, they have a Wildlife and Heritage Service phone number at<personal data hidden>. The phone line is staffed during regular business hours, 8:00 am to 4:30 pm, except state holidays and weekends.

Miri