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Bald Eagle Hatching Process

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We are opening a new thread for discussion about the hatching process.  A quick reminder: As has been stated in previous "Posting Guidelines," we ask that your posts and comments are limited to this topic in this particular thread.


In keeping with the sequence of events at the Decorah nest site this season, this thread is committed to the hours just before hatch as well as the hatching process itself.  As Finn mentioned in the Bald Eagle Eggs thread, during the second half on incubation, the embryo is no longer free floating but is now fixed in the shell so when it is turned, it will resume its equilibrium position.  The first slight pip of the shell normally occurs at the edge of the air cell.  A well located or positioned pip indicates that the chick is in a good hatching position.  According to Nick Fox, as the pip progresses and the egg tooth is raised, the actual hole will tend to be further to the right on the equilibrium line.  He goes on to say that when the chicks starts to hatch properly, the breaking up of the shell will continue right handed at the blunt end of the shell.

The parents sit on the egg(s) lightly but so purposefully and as we have noticed with mom and dad their attentiveness (time spent incubating) has been virtually continuous.  Their carefulness in toe and talon placement reduces chances of any damage to the eggs as well as preventing chances of the egg membrane from drying up.  So between mid incubation  and pip, the embryo is getting itself into the right position so it can successfully hatch.  During this incubation stage the embryo exchanges carbon dioxide for oxygen by diffusion through the shell pores into the vascular membrane.  Gas exchange increases  during the course of development  so that by the time the air cell is drawn the demand for chorioallantois, which acts like a placenta, reaches its peak. 

Here's an interesting fact!  The developing embryo absorbs about 6-7% of the calcium from the shell but it does not markedly change the porosity to gases.

About 72 hours before hatching the egg starts a sequence of events that are critical.  The chick must progress and change from allantoic respiration (the blood vessels linked to the embryo that provide  means of respiration) to lung breathing.  It must also retract the yolk sac and finally hatch.  During the time the air cell begins the draw down, the complexus muscle, (also known as the pip muscle or the hatch muscle) located at the nape of the chick's neck starts to swell and also begins twitching spasmodically.  This twitching movement travels through the chick's body and causes it to straighten, just briefly.  This straightened or unbending position causes the chick to press tight against the shell which then forces its beak upwards.  The head is up and at the larger end of the shell.  Located on the upper beak area is a little hard calcium protuberance called an egg tooth.  This temporary tooth begins to penetrate the inner shell membrane into the air cell.  Once the beak cuts into the air cell, the chick can start breathing air into its lungs but at this time it is still dependent on allantoic gas exchange.  Here is when the little chick starts to chirp which means its is now breathing air! 

(A side note on the complexus muscle: A few days prior to hatching, it starts to swell with lymph until it reaches a peak size at pipping time.  The purpose on the swelled muscle is to help brace and cushion the head as the chick forces the egg tooth through the shell. Then it loses the fluid in a few days and assumes the appearance of a normal muscle.  In adult birds it functions to raise the head.)

Now that the chick is breathing repetitiously, the air in the air cell becomes high in carbon dioxide which stimulates the hatching muscle even further.  With little tappings at the shell from the chick, a small area of the shell begins to lift.  This is called starring where tiny amounts of air enter.  Here is a link to a photo of what starring looks like at this point.

While working sporadically and resting the chick begins to break up an area around the pip and splits the air cell even further and gradually it enlarges the opening where the beak and the egg tooth can be seen.  This is the time when the little eaglet may rest for up to 24 hours!

Here is a photo of a pip

In Understanding the Bird of Prey, Nick Fox provides a chart of the events leading to hatch and the timeline, of which many overlap. 

Allantoic Respiration:  72 hours till hatch to approx. 6 hours to hatch

Internal Pip:   about 66 hours till hatch to approx. 36 hours till hatch

Lung Breathing:  About 48 hours till hatch and continuing

Chirping:  about 42 hours till hatch and continuing

Finn will discuss the 24 hours or so before hatch and the actual hatch itself

OH wow!!! Thank you for all of the fabulous information...I am still trying to wrap my head around what  chorioallantois  is....every article I have tried to read seems like GREEK to me!!! lol
This is such a miracle...and I am so excited to be able to watch it all unfold again!!!
I can't thank all of you enough for the limitless amount of knowledge that you share with us!!! Thank you all SO much!!!

Late afternoon April 1 I saw the eggs, and one had a hole large enough I saw the struggling eaglet inside.  :D
Earlier the parents had switched - but did not permit a view of eggs - so I just waited  (no, not patiently)
This is indeed eggciting

The Chorioallantois Membrane also known as CAM is a vascular membrane.  It is formed by the fusion of the chorion and the allantois walls.  Before the chick pokes a hole through the air cell in the egg the CAM supplies oxygen to the developing embryo.  Oxygen enters and leaves through the pores of the shell and into the blood in the capillaries of the CAM.  The blood circulates through the embryo and supplies oxygen before the lungs function.  I hope this helps explain the process  little more.

Here is a cross section illustration of an avian embryo.


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