We've assumed this meant when they were caged but what if it has a different meaning? Google that term and this was the answer:

It can start as early as 4:00 a.m. and last several hours. Birds can sing at any time of day, but during the dawn chorus their songs are often louder, livelier, and more frequent. It's mostly made up of male birds, attempting to attract mates and warn other males away from their territories.