Hanover Bald Eagle Blog # 5
Jan. 3, 2019
In partnership with Pennsylvania Game Commission and Comcast Business .
For those nocturnal eagle enthusiasts who are peeking at the nest cam after hours, you may have noticed that Liberty and Freedom are not sleeping at their nest. This is because outside of the active nesting season, bald eagles sleep at roost sites. Roosts are places where birds regularly settle to rest. Some species, including Pennsylvania’s avian scavengers (eagles, vultures, and corvids) are known to roost communally, although it is natural to see bald eagles roosting singularly or in pairs, especially during nesting season as other eagles have likely been driven out of the area. Liberty and Freedom are a perfect example, as they roosting in a tree close to their nest!
Several factors influence the choice of a roost site, and like many of us, eagles like to save energy whenever possible. Energetics (including food intake and regulation of temperature), as well as the exchange of information between birds are two of the most important site criteria.
Many social scavengers are adept at learning through observation where their neighbors may be finding lunch, and many scientists agree that communal roosting may help facilitate this process, allowing birds to save energy they would otherwise spend looking further for food. When you spot a plethora of eagles perched together during the day, they are likely taking advantage of a local food source such as a fish-laden body of water, or an ephemeral meal of road-kill deer.
Roost sites are also a partial solution to the issue of cold stress. Unlike us, eagles cannot turn to a wood stove or the comfort of slippers when they feel chilled, and loss of heat through convection is one of the most dangerous threats to raptors in the winter. Convection occurs when an animal’s skin, feet, or feathers touch cold air, and while eagle’s have impressively insulating feathers, strong winds can still cause exposure. This is why overnight roosts are often sheltered from the elements. Just as nest sites differ between regions, roost sites also vary depending on what is required. For example, protection from frigid winds shapes roost selection in Pennsylvania more so than in Florida.
Interestingly, there is a national eagle roost registry, a project started by the Center for Conservation Biology in 2007. The objectives of this study were to “provide roost locations to regulatory agencies, land managers, and other professionals to improve the protection and management of roosts.” Pennsylvania currently has 49 officially registered bald eagle roosts through this project, out of 1,538 registered in total (within the U.S and several Canadian provinces). Roosts are protected under the ‘disturb” clause of the Bald and Golden Eagle Protection Act, and as observers, we are asked to demonstrate respect when viewing roosting eagles. For more information about this project, including numbers of registered roosts per state, visit: https://ccbbirds.org/what-we-do/research/species-of-concern/species-of-concern-projects/national-eagle-roost-registry/
“Unlike nesting sites, most of which are in remote areas, many wintering grounds are close to large numbers of people. Human contact with the eagles is inevitable and disturbance of the birds a constant threat. Not to be overlooked is the fact that the use of favored roosting sites varies from year to year, depending on the availability of food. Thus, it is important to preserve as much of the natural ecosystem of these wintering areas as possible. Many are not protected at all.” -Eagle Nature Foundation, Apple River, Illinois
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