Any Day Now
We expect the eggs to begin hatching sometime within the next few days. Peregrine falcons typically incubate their eggs for 33-35 days. Incubation usually begins with the 2nd to last egg laid (May 29th this year) - but this year the start of incubation was a bit of an on again/off again affair, so pinpointing exactly when incubation began was a bit tricky. Regardless, we should see the first egg hatch soon.
The interior of the eggshell is lined with membranes that together form what is called the chorioallantois. The many blood vessels of the chorioallantois pass oxygen through the outer shell and to the developing embryo (while expelling carbon dioxide). Metabolic wastes are also transported to this membrane. Since being laid the egg has been slowly but steadily losing water through the pores of the shell, as a result the egg actually weighs less now then when it was laid. The shell is somewhat thinner now as some of the calcium in the eggshell has been absorbed by the embryo and used to help build its developing bones.
At this point the embryo's development is largely complete. The chick will reposition itself inside the egg bringing its bill close to the air space that occupies the end of the egg. The embryo is absorbing what remains of the yolk into its abdomen and will begin to absorb and remaining fluid in the egg as well.
The embryo pierces the air space with its bill and breathes with its lungs for the first time. Now the real work of hatching begins. The hatchling will use two specialized structures in this demanding process: the egg tooth a small, sharp structure on the upper beak of the young bird(this will fall off shortly after hatching) and the complexus muscle (also called the pipping muscle). This muscle is greatly enlarged prior to hatching and is used to brace the head as the egg tooth is rasped repeatedly across the shell. Eventually the hatchling breaks through or pips and a small hole will be seen at the end of egg. The hatchling will rotate within its shell, continuing to score the shell until it is has created an opening large enough to emerge through. The chick will rest frequently during this demanding process. The entire process can take days.
By delaying initial incubation the time between hatching of each egg has been reduced. All of the eggs in a clutch typically hatch within 24-48 hours of each other. This is important as it reduces the disadvantage the youngest chick faces in competition with its siblings
The parents can hear and feel the activity inside the egg and may respond to it - especially at the later stages of the process. Once the hatchling has begun to breathe with its lungs it can also begin to make soft calls which the parents can hear and may become louder as hatching progresses. We have already noted the adults peering closely at the eggs over the last day or so.