Hanover Bald Eagle Blog #18 - 2021
May 03, 2021
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Bald eagles projectile poop. It’s not pretty, it’s not subtle, but this behavior actually keeps the nest sanitary. Well, as sanitary as a nest filled with carcass remains can be.
Birds do not chew food like us; they swallow food whole or use their beaks to rip, crush, or break items into smaller pieces. Some birds such as ravens, crows, and gulls have discovered innovative ways to initiate this process by dropping shellfish on rocks, or in the case of black vultures, dragging coconuts into roads to be flattened by oncoming cars. After a bird swallows, the food moves down the esophagus and into a crop, where it’s stored so that the bird has more time to digest. Once a morsel passes on to the stomach, it goes through the proventriculus which softens the food, the gizzard, which grinds it, and then on to the intestines. Next is the ceca, two pouches that assist the bird in absorbing remaining water and completing the final steps of digestion.
Finally, it’s time for expulsion of the remaining components, bringing us to the apt term well known among birders: whitewash. Bird poop can serve as a helpful clue to the whereabouts of roosts, perches, and other bird hangouts.
The Hanover eaglet would quickly soil the nest if they “went” inside the nest, but as we’ve seen on camera, they back their booty up to the edge of the nest, aim, and shoot. This behavior is a behavioral adaptation that facilitates nest sanitation and relieves the Hanover parents of the unappealing task of removing the feces themselves. Many other bird species have to do just that: songbird chicks, for example, produce fecal sacs soon after digesting a meal, and their parents quickly pick up the sacs with their beak and either eat them or drop them far from the nest. This is important, because the accumulation of feces could attract predators.
Bird poop contains two components: metabolic waste (excretion) and food waste (defecation). When an animal eats, the food is absorbed into the body and used to build proteins, store energy, and facilitate other crucial life processes. These actions produce nitrogenous waste, which the body must expel before it becomes toxic. To get rid of this waste, mammals (including us) dissolve the waste in water and then urinate it out. Birds take a different approach and for good reason. Some birds migrate long distances and finding freshwater is often a problem, especially for species crossing oceans. Birds also need to expel waste inside the eggshell and urinating is not an option given the confined nature of an egg.
Therefore, birds use uric acid instead of water to dissolve waste. Uric acid is easily stored inside the egg. The white part of “whitewash” is a result of uric acid.
Even with these basics, not all bird poop is created equal. Piscivorous (fish-eating) birds like bald eagles produce whiter poop than many other species because fish are easier to digest and absorb than prey items that contain hair or exoskeletons. In other words, when an eagle digests a fish it produces less food waste which is the dark component of bird poop.
Sanitation is not a universal priority among birds. Burrowing owls, for example, repurpose old mammal burrows for their nests, and research shows that owls are more likely to return to a mammal burrow that still contains old material like dung. There may be benefits to living in a dirty home after all.
Remember: When birdwatching, one should keep their mouth closed, no matter how jaw-dropping a bird sighting might be, because at any point, whitewash may fall from the sky.
Golden eagle nests in Alaska are sometimes ringed with orange lichen and grass tufts, because eaglet poop creates a fertile environment for things to grow that wouldn’t otherwise exist on those rocky cliffs. Researchers look for these poop gardens when they survey for nests from the air.
Riding, C.S., & Belthoff, Jr. R. (2015). Removal of old nest material decreases reuse of artificial burrows by burrowing owls. Wildlife Society Bulletin, 39(3), 521-528. http://doi.org/10.1002/wsb.552
Opar, Alisa. (2011, June 29). What do Nesting Birds do with all that poop? Audubon. https://www.audubon.org/news/what-do-nesting-birds-do-all-poop
Mayntz, Melissa. (2018, December 24). How Do Birds Eat? Digestion Facts. The Spruce. https://www.thespruce.com/digestion-facts-for-birds-386437
ATTENTION HANOVER EAGLE VIEWERS - We recognize that over the years this bald eagle pair has been named by the public and is commonly referred to as "Freedom" and "Liberty". While we understand that naming the eagles helps connect and distinguish the female from the male eagle, naming the pair introduces an element of domesticity to wild animals. In order to respect the eagles and focus on their natural history, we will refer to the female and male as such as per recommendations of the Pennsylvania Game Commission.
RAPTOR ECOLOGY SPECIALIST - ZOEY GREENBERG
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