Temperature, humidity and regular rotation are important to ensure the survival of the embryo and possible hatching. In 1969, H. Lundy identified five temperature zones characterized by their effect on the developing embryo. These studies were conducted on chicken eggs in an artificially controlled environment, but we will look at them to see how eagle eggs could react to ambient temperature.
These five zones are:
Overheating or killing zone: above 40,5 ° C.
The optimal temperature, the so-called hatching potential zone, is 35-40,5 ° C. The American Eagle Foundation reports this temperature as 37 ° C for bald eagle eggs incubated in an artificially controlled environment.
Excessive development zone: 27 to 35 ° C. Embryos in eggs that spend too much time in this zone may develop unevenly, leading to injury or death. Successful hatching is significantly reduced.
Delayed development zone: -2 ° C to 27 ° C. Eggs do not develop at this temperature. Freshly laid eggs can spend a lot of time at this temperature without damaging the egg or embryo.
Zone of injury or cold death: below -2 ° C. Although eggs contain large amounts of water, they can survive in the cold below 0 ° C. However, eggs that reach -2 ° C will freeze, which usually causes death. Some exceptions to this rule have been observed in mallard ducks.
What does it all mean?
In the last days before laying eggs we can see how the female and the male put the bowl on the nest or a cup of soft dry grass or corn rustle. (depending on the area). The grass adapts to the eggs and is wrapped to provide warm dry insulation below and around them. The female and the male transfer heat from their bodies and regulate the temperature by alternating sitting on the eggs or standing by them. Sitting low in the nest helps keep eggs and incubating parents warm. As the cold deepens, eagles bring more grass for insulation.
What is the eagle's temperature?
In a study of wintering a bald eagle, Mark Stahlmaster found that the average body temperature of adult bald eagles ranged from 38,9 ° C to 41,2 ° C. daytime had an effect on body temperature. Since the heat damage zone may be lower than the body temperature of the incubating adult, it is easy to understand why eagles could (in warmer weather) spend time away from their eggs.
Although males accept their share of egg duties, larger females spend more time incubating in extremely cold weather or at night when eagles can lower body temperature to reduce energy loss. Although the female is not so much larger than the male, it may mean a difference in cold weather.
Lundy's work was based on artificial hatcheries where temperature and humidity can be strictly controlled.
Eagles live in the real world, subject to real weather conditions. Yet they have an excellent reputation for hatching eggs. They spend a lot of time incubating on their eggs and apply the heat from the very warm bodies directly to the eggs. Unlike us, they don't need instructions to provide information on temperature regulation, egg rotation, or care.
Sherri Elliott mentioned the difference in egg incubation - the female incubates the eggs with a triangular pattern, while the dad incubates them in one line. This is almost certainly related to the difference in size between the two (and the overall area of the breeding patch), although "shapes" are not uncommon. Just like when sitting and standing, shapes and locations can help regulate temperature and keep eggs alive.
The incubation patch begins to develop on the chest or abdomen of both the female and the male shortly before the female can lay eggs. Due to hormonal changes, the feathers covering this area fall out, leaving a wrinkled piece of bare skin where the blood vessels fill with warm blood.
When we see that the female or male "twists" on the egg adapts the egg patch to keep it warm.
Source: Introductory photo of Decorah North nest, bald eagle - Shelly Fowler, https://raptorresource.blogspot.com