Hanover Bald Eagle Blogs # 22 - 2021
June 1, 2021
In partnership with Pennsylvania Game Commission and Comcast Business .
ON MEMORIAL DAY WEEKEND, SUNDAY MAY 30TH AT AROUND 12:20PM EST THE HANOVER EAGLET, H313, FELL FROM THE TREE. THANKFULLY GROUND VIEWERS WERE ABLE TO SPOT THE YOUNG EAGLE ALIVE AND WELL ON A NEARBY LOWER BRANCH. BY THE END OF THE WEEKEND, WE'VE HEARD REPORTS OF THE EAGLET SEEN FLYING WITH BOTH PARENTS TOWARDS THE LAKE. A PICTURESQUE ENDING TO THIS 2021 SEASON!
I AM SURPRISED BY THE SIZE OF THE LONE EAGLE. IS THEIR SIZE A RESULT OF HIM/HER BEING AN ONLY CHILD?
Very little research has been done on the difference between nestling growth rate for solo eaglets versus eaglets with siblings. That being said, the Hanover eaglet’s large size could be a result of them having no competition for food, and/or being a female. As mentioned in previous blogs, we can only speculate as to the sex of the eaglet, and we could be wrong either way. Studies have shown that bald eagle siblings can and will kill each other (called siblicide), and in most cases, the first-hatched eaglet ends up being the strongest and healthiest of the bunch because they have a head start on growth and they outcompete their sibling for food. Bald eagles hatch asynchronously, meaning there is an interval of several days between each egg. See Blog #24 from 2019 for more information on the theory behind this hatching sequence. Without a sibling to worry about, the Hanover eaglet had more consistent access to food and this could very well be linked to their robust size.
FUN FACT: THE DAILY WEIGHT GAIN FOR NESTLING BALD EAGLES IS THOUGHT TO BE HIGHER THAN ANY OTHER NORTH AMERICAN BIRD, AT ROUGHLY 102 GRAMS PER DAY FOR MALES, AND 130 GRAMS PER DAY FOR FEMALES. FOR REFERENCE, A STICK OF BUTTER WEIGHS 110 GRAMS.
IF THERE IS ONLY ONE EAGLET IN THE NEST, DOES IT TAKE LONGER FOR IT TO FLEDGE?
I could not find any research addressing this question. It seems unlikely that a solo eaglet would take longer to leave the nest, since they have access to more food than they would if they shared the nest with a sibling and could therefore grow more quickly. That being said, the Hanover eaglet has not yet fledged, even though it’s been 11 weeks since time of hatch. The window for fledging is typically between 8-14 weeks. So, for this eaglet at least, it’s hard to say whether having no siblings has impacted their fledging timeline.
WHY DO EAGLES EAT COMPOSTED NEST MATERIAL?
When we see the eagles picking up nest material in their mouth and moving it around, they are not eating it but rather airing out the nest, theoretically to help reduce bacteria build up. They may also be re-lining and re-cushioning the depression the egg sits in, called the bole.
DO EAGLES REMOVE FISH SCALES BEFORE THEY EAT THE FISH?
No — they rip into the fish with their powerful beak rather than removing scales, often beginning with the head of the fish, especially if they are young nestlings just beginning to feed themselves. Their stomach acid breaks the scales down and/or the eagle’s cough them up into pellets.
HOW LONG HAS THE NEST CAM BEEN UP?
Since 2015.
RECORD FOR LONGEVITY OF A NON-VIABLE EGG?
I could not find a record for how long a non-viable egg has remained in the nest. The record for the longest period of time before an egg hatched was 44 days. The typical “hatch window” for bald eagles is between 34 and 38 days, with 35 days as the average. Most viable eggs hatch within this window. This year the unviable egg sat in the nest for 79 days, on April 23rd we saw the egg was crushed by the eaglet in the nest at about 11:07pm. There are some reports of the eaglet eating the egg and the parent finishing up the shell later the next morning.
IS THE FIRST EAGLE EGG LAID USUALLY A FEMALE?
Yes, research has shown that there is a 93% chance that the first egg in an eagle brood will be a female.
WHEN AN EAGLE IS LEFT ALONE IN THE NEST IS IT ABLE TO DEFEND ITSELF FROM INTRUDERS?
It depends on the age of the eagle. For the first three weeks of the nestling’s life, a parent is present at the nest nearly 24/7. After 5-6 weeks, the parents roost close by, but their presence inside the nest drops off. The parents wouldn’t leave the nestling unattended if it was a risky choice, and in fact nestlings hardly respond to their parent’s alarm calls, suggesting that there isn’t much for them to worry about. Nestlings of other bird species usually hunker down when their parents sound the alarm, blending into the nest as best they can to avoid detection. When a bald eagle intruder shows up in the Hanover territory, one of the parents usually chases them off long before they approach the nest.
The feet and beak of a nestlings develop quickly, but mostly for defending against aggressive siblings. Against an adult bald eagle intruder, or a great horned owl, a nestling is likely defenseless until their motor skills, coordination, and muscles are developed. At this point, the Hanover eaglet would make a formidable opponent, and I, for one, would not mess with them!
Bald eagle eggs and nestlings are sometimes eaten by predators, though bald eagles face fewer nest predators than many other bird species. Bald eagle nestlings are sometimes killed by black bears, raccoons, hawks, owls, crows, ravens, and there are even reports of a bobcat and a wolverine taking a nestling. Fledglings (a term referring to an eaglet that recently left the nest) are actually more vulnerable than nestlings. If their first flight is unsuccessful — and up to half of first flights are — the fledgling remains on the ground until their flight feathers and muscles fully develop. This grounding period can last for a few weeks, during which fledglings are vulnerable to ground predators.
WHAT IS THE SIGNIFICANCE OF A BALD EAGLE RAISING THE FEATHERS BETWEEN THEIR SHOULDERS?
Bald eagles have feathers called hackles situated on the back of the head. When an eagle is threatened or aggravated, they raise their hackles as a warning to whomever they are peeved at. We see this behavior during tussles over food at carcasses, between siblings in the nest, and when intruders are around.
WAS IT NORMAL FOR THE EAGLET TO ROLL THE UNHATCHED EGG AROUND, INCUBATE IT, AND MOVE NEST MATERIAL AROUND?
Yes. There are not many reports or studies available on how solo nestlings respond to an unhatched egg, so in this sense, the Hanover nest cam provides a new lens into a unique circumstance. Mimicking is a central part of the learning process for young raptors, especially social species like bald eagles. The Hanover eaglet watches the parents closely and will continue to do so after fledging. Catching fish and scavenging are going to be important learning curves for the Hanover eaglet and they will learn as much as they can from their parents before dispersing. Even after they gain independence, they will continue to learn from adults at communal roosts.
THANK YOU TO ALL VIEWERS WHO ASKED QUESTIONS, AND TO ALL WHO READ THE ANSWERS! YOUR PARTICIPATION IN THE PURSUIT OF EAGLE KNOWLEDGE IS DEEPLY APPRECIATED, AND MAKES THE NEST CAM FEEL LIKE THE TRUE COMMUNITY THAT IT IS.
SOURCES
Bortolotti, G. (1984). Physical Development of Nestling Bald Eagles with Emphasis on the Timing of Growth Events. The Wilson Bulletin, 96(4), 524-542. Retrieved May 24, 2021, from http://www.jstor.org/stable/4161989
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. https://doi.org/10.2173/bow.baleag.01
Gerrard, Jon M. and Bortolotti, Gary R. (1988). The Bald Eagle, Haunts and Habits of a Wilderness Monarch. Smithsonian Institution.
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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