Today March 5 at Norfolk Botanical Garden photographer Shelly Fowler captured an outstanding image of the incubation patch – also known as the brood patch – on the female bald eagle. So just what is the brood patch and do all birds develop one during the breeding season? No – not all birds develop a brood patch. Some birds like the blue-footed booby use their feet in place of a brood patch. Some birds like gulls and plovers have three brood patches, and other birds like pelicans and cormorants do not have an incubation patch at all. Usually only the sex that incubates the eggs gets a brood patch such as male phalaropes do but the females do not because only the male incubates their eggs. In such species as the bald eagle, they both develop an incubation patch because, as we see each day, they both share the incubation duties.
The incubation patch begins to develop on the breast or abdomen shortly before the female lays her eggs through hormonal changes that cause the feathers that cover that area to fall out on their own. That leaves a wrinkled patch of bare skin that blood vessels fill with warm blood. When we see the female or male “wiggle” as they settle upon the eggs, they are spreading that bare patch over the eggs to keep them warm.
We should begin to see eaglets in about a week. I have spent time this week reviewing production records from the annual bald eagle surveys taken in Virginia by The Center for Conservation Biology. Only one other pair of Virginia bald eagles – in Prince George County (nest PG0602) – has consistantly produced three eggs like this pair at Norfolk Botanical Garden!! We have really been priveleged viewers.
Copywrited photograph used with permission of Shelly Fowler.