All is looking good with the 3 chicks while we wait for the 4th egg to hatch. Durand has switched from incubating posture (where she is tight down on the eggs) to brooding posture. At this stage the chicks can't regulate their own temperature so it is necessary for an adult to keep them warm. When brooding, Durand will sit higher up over the chicks and cup her wings around them as seen in the photo above. This different posture is necessary to not smother the chicks but it also works out that the chicks are larger than the eggs and certainly more squirmy so Durand doesn't have much choice other than to sit higher! In most birds of prey, the females are actually larger than the males. It is during brooding that this size difference comes in handy! Although Spark can and does brood the chicks while Durand is taking a break, it is certainly easier for her with her larger body size.
If viewers are watching and listening, then you have certainly heard the chicks chirping off and on. Interesting to have more activity in the nest in these last few days then we've seen during the past few weeks during incubation.
Unfortunately, we don't know the exact date the last egg was laid because the streaming video was down at that time. But I have in my notes it was between 3/18-3/21. Counting forward 33 days means that final egg could hatch at any time. Or, Durand and Spark could repeat their last couple of years and only have 3 young in the nest. Only time will tell.
EDIT at 4:10 pm: The 4th egg IS hatching! Durand is keeping it covered up but a brief glimpse did show a crack in the egg!