This video is about nesting tree swallows in Maine, USA.
From NestWatch in the USA:
Tree Swallows Share Nest
NestWatcher Ian Stewart discovered something unusual in a nest box in Greenville, Delaware, this past season. Two female Tree Swallows laid their eggs in the same box, for a whopping total of 10 eggs! Instead of abandoning this full house, both females appear to have stayed to incubate the eggs and feed the young. Although we cannot know for sure if two different females fed the young, because the birds were unbanded, it’s very unlikely that one set of parents could raise a brood this large without help.
All 10 eggs did hatch, although the last one was one or two days later than the others and did not make it. Several weeks later, nine baby Tree Swallows fledged from this nest box, a remarkable outcome for a nest that, under different circumstances, may have been abandoned by both females. See more photos from Ian here.