An interesting detail is that the two remaining birds continue to be brought food by the parents. I had always read that the way adult birds coerce their young to fledge is by withholding food, but I didn't really see any withholding going on with this group.
Here's the video of the first bird fledging. Based on other take-offs that I've watched (of "our" birds from last year), this little one is very matter-of-fact about the whole process, and leaves just like it's the most natural thing in the world! (At the beginning you'll note that one of the parents feeds one of the babies and takes away a poop sac.)
The second baby left in much the same manner as the first, following his or her lead and just jumping right on out.
The third baby, however, decides to use the box that contains the camera as a launching platform before exiting to the bottom left of the screen. Watch how the remaining two birds "talk" to their departing sibling:
Baby number four made a noble, yet unsuccessful, attempt at leaving. Lots of flapping, but it couldn't quite bear to let go of that nest yet!
Let's see how long it takes for those last two birds to really stretch their wings and fly away!