Hanover Bald Eagle Blog #22
May 3, 2019
In partnership with Pennsylvania Game Commission and Comcast Business .
While the Hanover nestlings engage in a variety of activities, we can all agree that a large portion of their time is spent catching z’s. They feed, sleep, defecate, and then sleep some more.
However, not all forms of sleep fit into our human conception of the activity, illustrated by the fact that animals require different lengths of REM sleep per night. For example, armadillos need 18 hours, humans typically need eight, yet a horse needs only two. Hypothetically, those species that spend the most energy would require the most recuperation time, and as many of us can attest to, human functionality drastically decreases the more sleepless nights we endure.
Many birds face similar consequences if they do not get enough shut eye, however there are some species that are remarkably adapted to handling sleep deprivation such as the pectoral sandpiper. Because these sandpipers have a polygynous society in which males mate with as many females as possible, and they breed in the Arctic circle where there is nearly constant daylight in the summertime, the males have evolved the ability to essentially skip sleeping for the benefit of actively enticing females for as long they possibly can (we’re talking continuous activity, 95% of the time, 19 days in a row)! The most active males are likely those that will sire the most offspring, and therefore any groggy side effects are well worth the effort.
Large raptors are typically monogamous, and therefore their mating escapades are not quite as energetically demanding, and they face a great deal less predation risk compared to smaller species. Both of these realities afford them more ease in their sleep habits since they don’t need to worry about resting out of sight of prowling predators. Bald Eagles can rest essentially anywhere strong enough to support them, anytime they want. The exception to this privilege is during the breeding season.
Even though few predators will mess with a sassy adult Bald Eagle, eaglets are another story, as we saw this week with the Great-horned Owl visiting the nest in the middle of the night, presumably looking for an easy eaglet entrée. Great-horned owls are ambitious generalists who will eat just about anything.
Unihemispheric sleep is an adaptation employed by some birds as a tactic to avoid becoming a snack, or, as is likely the case for the Hanover eagles, to avoid being caught off guard. During unihemispheric sleep only one hemisphere of the brain rests at a time, an adaptation thought to allow the sleeper to remain partially active, while still engaging in needed recovery time. To make this act even more confounding, one eye is kept open on the side of the brain that remains active. Whales and dolphins are well-known for this phenomenon, in part because it allows them to remain synchronized as a pod even during periods of rest.
While unihemispheric has not been verified for eagles specifically, it likely serves as a useful adaptation during the breeding season for a variety of birds as it would allow breeding adults to keep a literal eye out for danger while raising nestlings. Even though Liberty and Freedom often look like they are sleeping, as those of you who have parented will understand, it can be difficult to truly rest while raising newborns. Often Liberty or Freedom stand up in the nest with eyes closed and head nodding whenever the nestlings are immobile enough to allow for it, which may not be long.
When sleeping at designated roost sites, Bald Eagles and other raptors often stand on one leg with their beak tucked into warm downy feathers. It may seem surprising that a bird can sleep on one leg, but in fact when weight in placed on their one foot, tendons tighten and help the foot remain tightly closed for as long as necessary.
Sleep composes roughly 1/3 of an adult bird’s daily activity budget, though naturally, developing young birds require more sleep than their parents. Human newborns spend about half of their time engaged in rapid-eye movement (REM) sleep while once we become adults, this shifts to about 25%. Broad-winged Hawk nestlings have been observed to engage in some form of sleeping posture for 97% of the time until about three weeks old. In a study conducted on barn owls, a similar shift was observed using an electroencephalogram, or EEG. This device is used in a variety of sleep studies to measure changing electrical activity levels in the brain. These shifts can eventually be recorded as brain waves, indicating varying levels of alertness, consciousness, and thinking. In this case, the EEG revealed that baby barn owls do indeed require less REM sleep as they age.
There is still a great deal of mystery within the dream world of raptors, and in years to come we may have the opportunity to learn more, especially with dedicated cam-watchers such as yourselves. Keep those observation skills attuned, and be sure to note whether you catch any of the Hanover eagles with only one eye open. You never know when they might switch hemispheres!
A NOTE ON BRAVEHEART AND BRIGHTEYES (THE LOCAL HANOVER SQUIRRELS):
As many of you have noticed over the past few years at the Hanover nest, rodent visitors occasionally flash across the camera screen, either diving headfirst into the substrate at the bottom of the eagle nest, or scurrying up nearby branches. The interactions between breeding eagles and their nest visitors has scarcely been described or studied, in large part without 24-hour scrutiny, the chances of witnessing such occasions are quite low. Therefore, your observations of these occurrences are valuable, and while we would love to provide cause-and-effect speculations on how these mammalian visitors interact with bald eagles on a scientific basis, most information is currently anecdotal. This being said, the stick-nest structure of the Bald Eagle nest provides many nooks and crannies similar in form to a squirrel nest, just more spacious. It is no surprise that these rodents are testing the water as tenants, and who knows maybe the furred and the feathered are coexisting just fine!
Yet another reason to keep your eyes peeled and take note of what you see; you never know when your observations make come in handy.
There are, however, intriguing accounts of non-raptors nesting near raptors for protection from specific predators. For example, in the Arctic, researchers have observed both snow geese and red-breasted geese nesting near snowy owls and peregrine falcons for what seems to be an increased likelihood of protection against the Arctic fox, a common predator of ground nesting waterfowl. Black-chinned hummingbirds in Arizona have also been observed nesting in higher densities near Cooper’s hawk and Goshawk nests, seemingly to avoid predation from Mexican Jays.
SOURCES
Athabasca University (2016, Oct 20). EEG Patterns in Sleep and Wakefulness. Retrieved from
https://psych.athabascau.ca/html/Psych289/Biotutorials/20/intro.shtml?
Bildstein, Keith (2017) Raptors, the Curious Nature of Diurnal Birds of Prey. Ithaca, NY: Cornell Lab of Ornithology Press.
Cornell Lab of Ornithology (2019). Do Birds Sleep? Retrieved from http://www.birds.cornell.edu/k12/do-birds-sleep/
Learn, Joshua Rapp (2015, September 8). Hummingbirds Benefit From Nesting Near Hawks. Retrieved from https://wildlife.org/hummingbirds-benefit-from-nesting-near-hawks/
Lesku, John A & Rattenborg, Niels C (2014). Avian Sleep. Retrieved from https://www.cell.com/current-biology/pdf/S0960-9822(13)01238-4.pdf
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