Most are situated high in the tree canopy, safe from predators on the ground. The female heron lays three to seven eggs in March and April. Both parents care for the eggs during the 28-day incubation period, after which both feed regurgitated fish to the newly hatched nestlings. After about 60 days, the young fledge and leave the nest. Because herons often congregate in nesting rookeries, scientists are able to count the number of herons that inhabit the Bay region each year.
Some rookeries are exceptionally large and are important preservation sites.
The Bay provides ideal food and habitat for the blue heron's survival. In 1990 surveys found 4,600 nesting pairs of great blue herons in more than 38 colonies in Maryland. At the Nanjemoy Creek Sanctuary near the Potomac River, volunteers counted more than 1,300 nests in that year. The area is under the stewardship of the Nature Conservancy. Herons have not always been so well protected.