Incubating the eggs: everything you need to know

Each day we have new questions about what we're seeing on the nest. After yesterday's excitement with the arrival of the third egg, we now wonder what will happen next. Will there be a fourth egg, or will incubation begin?

It is highly unlikely that there will be a fourth egg. The usual egg count is almost always two or three. John Blakeman has sent us below all we need to know about the next stage - incubation - when the formel will be spending hours at a time on her eggs.



John Blakeman tells us:

"The laying of the third egg will initiate sincere, authentic developmental incubation. Until now, the formel has been sitting loosely on the eggs. She has merely kept them slightly warm, not the prolonged and enduring 100 degrees F (or so) temperature needed for incubation.

Until now, sitting has been merely maintenance. The embryo of the first egg has not developed much, if at all. A fertile egg kept at the start between 40, 50, or 60 degrees or so simply just sits there, viable and alive, but with no growth of the organism inside.

At this point the baby hawk, which is called an eyass (EYE-ess) not a "chick" by those who know raptors, is only a mass of undifferentiated cells in the blastodisc on the surface of the yolk.

With proper incubation now beginning at consistent warm egg temperatures, the blastodisc cells will begin to grow, using the lipids in the yolk for energy, and the proteins and minerals in the egg white as building materials for the new tissues and organs.

The egg, or at least the developing embryo, will now start to metabolize, to take in and use oxygen and give off and expel carbon dioxide. Of course, the egg and developing embryo have no functioning lungs or other structures to accomplish these gas exchange processes. Oxygen goes in and carbon dioxide comes out of the egg through the shell, which on a microscopic level is rather porous.

In the next 28 days or so the egg will actually lose weight, as water evaporates out through the shell. The rate of this water loss is important and can signal the rate of development of the embryo and eyass inside. Rapid loss of water will kill the egg, and no loss of water would indicate that the embryo is already dead. The moist skin of the formel’s brood patch (bare skin on her lower chest/belly area) will help to control the moisture in the egg.

But these should not be concerns. This is an experienced, successful pair of red-tailed hawks who incubated, fed, and fledged three eyasses last year. They’ve done this before and know what to do, so we needn’t be concerned.

Here are some things to look for, and one thing not to be concerned about.

First, only the formel has a distinct, naked brood patch under her chest and belly feathers. The tiercel retains his downy feathers, so when he sits to incubate during the times the formel gets up, stretches, preens, defecates, and otherwise attends to herself, the eggs cannot be kept quite as warm as when they are in direct contact with the formel’s naked brood patch skin. This is one reason the formel does most (but not all) of the incubation. She’s just much better equipped, both physically and mentally, for this important job.

And to do that, watch now how she settles on the eggs. Until the third egg was laid, she and her tiercel just sort of plopped down gently on the eggs. Now, once she realizes that another will not be coming (red-tails in the Midwest and East virtually never produce four eggs), she will be much more diligent in settling upon them. She will sink a bit more slowly, and will often gently rock back and forth to position the eggs right up against her brood patch.

I have to think that this is a profoundly satisfying and rewarding feeling for the mother, to have those eggs right against her body. After she’s been away and resumes incubation from her colder mate, I’m sure she feels the slightly cooler egg temperatures. It must be gratifying for her to get them all cozy once again.

And that’s the second thing observers will note. From time to time, the eggs will be left unattended out in the cold air, often for 10 to 20 minutes or more. We’ve seen this already, many times. But true incubation hadn’t started yet, so there was no problem. Now, with development of the embryo in the blastodisc, an overly long period of cooler temperatures will kill the eyass.

All of us who have watched red-tail nests have been really anxious about these long, egg-cooling intervals. But in virtually every case, except with first-year or inexperienced adults who do often fail to keep the eggs warm enough, experienced adults like these know just when to get back on the cooling eggs and resume real incubation.


Why, then, do the parents allow these unattended, periodic cooling periods? We don’t know. I speculate that it may have something to do with oxygen diffusion and absorption into the liquid tissues of the egg and embryo. As you might have recalled from high school chemistry, the warmer a liquid is, the less gas remains dissolved in it. You didn’t drink that warmed-up soda after the carbon dioxide diffused away from it as the soda got too warm.

So, it’s possible that once a day or so the parents allow the egg to cool a few degrees for 10 or 20 minutes, allowing more oxygen to diffuse through the shell and become dissolved in the egg fluids. A breath of fresh air, as it were, for the developing eyasses.

Or, perhaps it just doesn’t make any difference with the oxygen, and the egg grows well enough with these periodic uncovered periods.

Either way, don’t be concerned until the egg remains uncovered for 45 to 60 minutes or so."

--John Blakeman