Hanover Bald Eagle Blog # 3 - 2020
Feb. 28, 2020
In partnership with Comcast Business and Pennsylvania Game Commission .
This week Liberty and Freedom have been diligently incubating the Hanover egg, constantly keeping the developing embryo warm. For many raptors, males and females share incubation duties early on in the nesting season. There are, however, other strategies employed by raptors whose life histories differ from bald eagles.
Sharp-shinned hawk females, for example, do all of the incubating. This is presumably because the male is so small that he cannot cover the eggs and keep them adequately warm. Northern harrier females also do all of the incubating, however the reason for this is less about size and more about the bold plumage of the male; were he to sit on the eggs, he would draw unwanted attention to the nest. Harriers are also often polygamous, and therefore raising young is already a single-parent endeavor.
Another incubation method is for sexes to share duties equally. This is most notable in species of Gyps vultures (black vultures, for example), whose food is more suitable for self-foraging. The female and male feed themselves during their “off” shift.
A third incubation method is for the male to relieve the female when he delivers food to the nest, and subsequently incubate while she rests and feeds. This is the pattern typical of most raptor species, including bald eagles. In the early stages of incubation, the male’s shifts last anywhere from 1-3 hours. Over time, his duties will lessen, and he becomes a full-time provider.
The importance of incubation is to provide warmth to the eggs, shade them from harsh sunlight, and protect them from predators. Liberty and Freedom have a high chance of achieving all three of these tasks if someone is constantly with the eggs. Mere presence, however, is not all it takes. There are several additional incubation behaviors that are essential to the healthy development of an embryo.
Liberty and Freedom must assess the egg and correctly determine when to roll and turn it. If they guess wrong, the embryo can perish, especially the early weeks of incubation. This is because the contents of the egg are oriented in a way that allows embryonic development, yet maintaining that orientation requires help from the parents. Liberty and Freedom must roll the egg roughly every two hours to prevent the yolk from sticking to the shell, which can kill the embryo. They also turn the egg once an hour, and shift their own position to help distribute heat evenly across the surface of the shell. The embryo must be kept at a temperature of around 105∞F (41∞C).
Prior to laying, Liberty occasionally plucked some of her breast feathers in anticipation of the egg’s arrival. She was creating a brood patch, a bare section of skin that increases the efficiency of heat transfer from parent to egg. Both males and females have brood patches, but since females eventually transition into the primary incubators, their patches are more exposed. As Liberty and Freedom settle upon their egg, you will notice that they plant their beak into the substrate and shift back and forth until they find a satisfactory position. This behavior parts the breast feathers and exposes the brood patch so that skin and eggshell are directly touching.
Another challenge for parents during incubation is to avoid stabbing the egg. This is not as simple as it seems, given that raptors are endowed with razor sharp feet and a built-in knife on their face. You may see Liberty or Freedom turning the egg with their beak open, or walking around the nest with their talons clenched into a ball. Both of these behaviors are protective measures so that no accidents occur.
At this early stage in the Hanover nesting season, whoever is not on incubation duty is likely perched nearby or hunting for food. Liberty and Freedom occasionally vocalize preceding shift changes, presumably to get each other’s attention or remind the off-duty parent that it’s their turn. As they transition, both eagles carefully step around the egg and gracefully trade places. Sometimes, the incubating parent will leave and the egg will be left unattended for a few minutes. This is normal and does not pose a threat to the egg.
Additional nesting material may be delivered at this stage, providing extra insulation for the incubating parent to tuck around their body, further trapping heat and adding cushion to the nest (notice this behavior in the above clip).
Other behaviors to keep an eye out for are intermittent dozing, and seemingly random nibbling of nest material, which has been proposed by some as a boredom cure. However, this may also be a way for parents to prune plant material away from their space.
Incubation habits of raptors differ based on geographical location; for example, peregrine falcons nesting in the Arctic engage in similar “tucking and scraping” behavior as what we see at the Hanover nest, however peregrines are laying their eggs on cliff faces and therefore their buffers are intended to prevent the eggs from rolling off the edge of the precipice. Arctic species are also more concerned about their eggs getting cold, and spend less time away from the nest during the incubation stage than temperate species.
Keep an eye out for subtle changes in incubation behavior over the next few weeks. You may be surprised at the level of perception Liberty and Freedom possess when it comes to the needs of their egg. We can expect to see a hatchling around March 18th, 35 days from when the egg was laid.
SOURCES
Bald Eagle Incubation Facts. (n.d.). Retrieved February 24, 2020, from: https://journeynorth.org/tm/eagle/annual/facts_incubation.html
Newton, Ian (2010). Population Ecology of Raptors. London, United Kingdom: Bloomsbury Publishing PLC. (Original work published 1979).
Written by Zoey Greenberg
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