Hanover Bald Eagles Blog # 5 - 2020
March 13, 2020
In partnership with Pennsylvania Game Commission and Comcast Business .
Last week an immature bald eagle visited the Hanover nest during the night, an event fraught with mystery for several reasons. First, the visitor was a difficult bird to age given the infrared viewing. Second, Liberty and Freedom did not react as strongly as when past intruders have approached the nest in daylight (Liberty even rolled the egg in the presence of the visitor). This left many of us wondering whether or not the visitor was Stars or Stripes (last year's offspring).
The plumage of the visitor points to no. Instead, we were probably looking at a three or four-year-old, meaning if the bird was a relative of Liberty and Freedom’s, it would likely be offspring from 2017. The older age was suggested by the individual’s scalloped feather edges, the light eye, light cere, and white on the head.
Even though the visitor did not act very aggressive towards Liberty, she clearly wasn’t happy. Key signs that a bald eagle is stressed include screeching, reduced sleep, reduced preening, and changes in body language. Liberty kept her eye on the visitor, altered her position if the bird moved, and vocalized many times. Bald eagles are also very averse to flying or being active at night, perhaps explaining why Freedom did not show up, and adding merit to Liberty’s frustration.
If the visitor was kin, then it may seem odd that Liberty was stressed by the interaction. In eagle world, however, family ties don’t last long. Loose associations remain between parents and their young for one to two months while the latter learns how to be an eagle and continues to receive food. Once the practice stage is over, everyone goes their separate ways. Bald eagles breed every year when possible, so non-breeding “teenagers” don’t have the option of returning home to mooch off their parents, who will be busy raising another brood. There is no open invitation from parents to offspring saying “you are always welcome back.” Young eagles may return to their natal areal because they know it well, but in general juvenile dispersal is opportunistic based on available territories and food resources.
Therefore, any bald eagle that visits the Hanover nest (kin or not) will be unwelcome, especially during the incubation stage. Adults are more threatening than non-breeding visitors, yet both still elicit a stress response, or at the very least, annoyance.
In a study conducted in northern Minnesota in 1987, Mahaffy & Frenzel addressed the various factors affecting territorial behavior in bald eagles during the nesting season. They highlighted the importance of “internal state [of the bird], extent of previous “ownership,” [of the nest], “attitude” of intruder, type of activity of defending bird, weather conditions, and time of day.” During the interaction at the Hanover nest this week, we saw several of these factors at play.
-- The response to decoys was greatest when they were noticed from across a body of water. The authors speculated that this was because as fish-eaters, bald eagles nesting near lakes may consider the lake an extension of their nesting territory, since it contains their primary food resource. There also may be higher visibility of intruders across bodies of water, meaning that a nesting pair of eagles can see intruders easier if they are near a lake versus at a terrestrial site with trees and other obstacles obscuring their view.
-- They also speculated that eagle pairs nesting on land may have greater tolerance for disturbances than those near water.
These researchers ran their experiment in 1979, and then again in 1980, with the same results, demonstrating consistency in their findings.
This deep dive into the territorial behavior of bald eagles gives us some context for this week’s interaction and prepares us to make more informed guesses next time we see a territorial response from Liberty and Freedom. That being said, the internal triggers of behavior in wild animals will always remain mysterious because at the end of the day, we’ll can’t read their minds!
To take a deeper look at the incentives behind territorial behavior, the researchers wanted to determine which factors caused bald eagles to react the most strongly. To accomplish this, they got creative.
They put up bald eagle “intruder” replicas (including a papier-mâché bird that worked well) and monitored the territorial responses at 16 active nests. They found several interesting results:
-- Territorial defense was strongest during the first 2 weeks following hatching, which follows the trend seen in other raptor species of defense decreasing in intensity as the breeding season progresses. For example, right now at the Hanover nest, Liberty and Freedom are on higher alert then they will be when their nestling is ready to fledge.
SOURCES
Buehler, D. A. (2020). Bald Eagle (Haliaeetus leucocephalus), version 1.0. In Birds of the World (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA.
Mahaffy, M., & Frenzel, L. (1987). Elicited Territorial Responses of Northern Bald Eagles Near Active Nests. The Journal of Wildlife Management, 51(3), 551-554. doi:10.2307/3801267
Personal communications with Dr. Todd Katzner, Dr. Trish Miler, Dr. Laurie Goodrich, Bill Clark, Jon Kauffman, Nick Bolgiano & Don Bryant.
Thanks to all!
Written by Zoey Greenberg
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