This year the red-tails laid their eggs on March 11, 14, 18 and 21. This means that the eggs are already 29, 26, 22 and 19 days old. However because she did not begin really incubating until the third egg was laid, the first three all are closer to 22 days over development while the fourth is probably three days behind. Regardless all are more than half way through their development process towards hatching. Last year the eggs all hatched on April 14th but those eggs all were laid more than an week earlier than occured this year. I suspect that we are probably looking at late in the week of the 17th this year.
As hatching approaches, I thought I would provide a bit of information on how this process occurs:
The chicks now fill an increasingly large portion oif the interior of the eggs that was once occupied by the yoke and the albumen. A large air chamber known as an "air cell" is developing at one end of the egg. If hatching goes well, the chick will shift into a "tucking position" where it places its beak between its body and its wing and then extends its beak into the air chamber and inflate its lungs. Prior to this time, the embryo has receive all of its oxygen needs through a network of capillaries that connect to a membrane inside the egg known as the "chlorioallantois" which imports oxygen from and exports carbon dioxide to the outside world.
Once the chick has broken into the air chamber it will begin to peck at the interior of the air chamber working its way slowly around the edge. The first break through to the outside world is known as 'pipping." The process of pecking its way out of the egg can take a day or two. Chicks have a feature on their bill known as an "egg tooth" which allows then to break through the hard shell. They also have a special muscle on their neck known as a ""hatching muscle" that gives them the strength to accomplish this task. This muscle shrinks once the chick leaves the egg.
Again still likely a couple of weeks away...but we are getting closer!