On 12/29/23 observers of the Southwest Florida Eagle Cam in Fort Myers noticed a “dimple” or slight indentation on one of the two eggs, a change on the egg’s surface that suggested a hatch may be in progress. It was soon confirmed (by the pattern of stains on the 2 eggs) that this was Egg #2, laid on 11/27/23 at 13:44 (by my observation). By the evening of 12/29, after about 32 days of incubation, the eggshell had a crack and some splintering along the side. By the next morning, 12/30/23, the shell was splintering on several sides and at the end. The outer shell membrane was visible between the bits of shell. The shell splintering continued throughout the day, and more of the membrane became visible. Observers could see movement by the chick inside the shell.
I (and others) have seen this kind of “messy” hatch (as I call it) on Bald Eagle nest cams several times over the years. Two shell membranes are fused to the shell and hold the shell fragments together. They are supple but fairly tough, so they can be difficult to break through. If the shell splinters in several places and the chick has a hard time breaking it apart, the membranes dry out, making them leathery and even tougher to split open. Most of the time the chick manages to wrangle a big enough space to emerge through the shell and membranes. But this kind of hatch is harder and more work than the more normal hatch where the shell breaks apart cleanly. In a few cases I have seen, it has proven too much for the chick, which becomes weaker from the effort and eventually fails to hatch.
But this case at SWFL was unlike the other “messy” hatches I have observed. In a normal hatch, the shell begins to crack open a day or two before the chick fully hatches. A shell breaking at 32 days of incubation, as at SWFL, might produce a hatchling at about 33 or 34 days. The earliest hatch in my records of Bald Eagle nest cams occurred after about 34 ½ days of incubation, and that was a third egg of a 3-egg clutch (third eggs usually hatch more quickly than first or second eggs), so the SWFL hatch would be a record early. (Stats on hatch timings are here.)
This suggested the possibility that the second egg’s shell was cracking prematurely, a suspicion reinforced by other indicators. One membrane around the inside of the shell is crucial to the embryo’s development (the chorioallantoic membrane, or CAM). It is full of blood capillaries, by which it effects the exchange of oxygen from the outside and carbon dioxide from the inside through microscopic pores in the shell. This membrane also stores metabolic wastes from the embryo and sheds them after the chick has hatched. (See more details on hatching here.) By the end of the day on 12/29 at SWFL, blood appeared on the shell and the membrane, which almost certainly came from the membrane’s capillaries. That raised some significant concerns about the health of the chick inside.
As most observers know, a day or two before the external pip in the shell, a swelling muscle in the chick’s neck contracts and pulls up the chick’s head toward the blunt end of the shell. There, it encounters an air cell, a space between the inner and outer shell membranes that contains a small amount of oxygen. The chick’s pipping tooth, which had formed along the top of its beak about a third of the way through the incubation period, pierces the inner shell membrane into the air cell. This is called the “internal pip.” It exposes the chick to air for the first time, which prompts the chick’s lungs and its nine air sacs to finalize their development and begin functioning. Over the next couple of days the lungs develop the ability to inhale oxygen and exhale carbon dioxide, gradually relieving the respiratory function of the chorioallantoic membrane, which begins to shut down.
The blood seen on the shell and membranes on the SWFL egg means that the chorioallantoic membrane’s capillaries were still performing the exchange of carbon dioxide and oxygen for the chick, that the internal pip probably had not yet occurred, and that the chick’s own internal respiratory system was not yet functioning. Whatever caused the shell to begin breaking, the damage to the membrane interrupted the transfer of respiratory function to the lungs, and the chick’s system probably began to experience oxygen deprivation and carbon dioxide build-up. This weakened the chick and increasingly rendered it unable to break through the dried-up shell membrane.
As difficult as this has been to watch, we can all be thankful that the first egg pipped on its normal timeline and had hatched by early this morning, 12/31/23.