This video is called Eastern Bluebird Singing.
From the Cornell Lab of Ornithology in the USA:
First Recorded Bluebird Twins Found by NestWatch Volunteer
Last year an Eastern Bluebird laid three normal eggs and one large egg in one of NestWatcher Gerald Clark’s nest boxes. A few weeks later he had five nestlings in the box, and his finding became a scientific paper on the first recorded instance of twins in bluebirds. (The Lab’s Dr. Caren Cooper tackled just how rare an event this is in a blog post for the journal PLOS.) The finding is a direct example of how citizen scientists contribute to scientific discovery each time they participate. Try NestWatch this season!
Cavity nesting: here.