North American birds are absolutely in decline. From this article [1], the overall population has decreased by about 1.5 billion birds since 1970. For some groups of species, for instance, aerial insectivores [2], things are much worse. For instance, the population of bank and barn swallows, chimney swifts and common nighthawks has decreased by 70% over the past twenty years alone.

The Christmas Bird Survey is done after most birds have migrated and so captures data on the relative abundance of resident birds, which as a group, are doing better than birds in North America overall.

[1] https://www.theglobeandmail.com/technology/science/report-fi... [2] http://www.bsc-eoc.org/download/BWCwi08.pdf