A few years ago in winter, a downy woodpecker began to work on this birch tree looking for food, when it was still standing but already dead. In the spring of the following year, a pair of black-capped chickadees spent a considerable time excavating the cavity as a nest site. They worked endlessly on it, ferrying mouthful after mouthful of excavated material away from the site. But after much work, they abandoned the site. This autumn, the snag fell down and split into pieces. I picked up the bit with the hole (cavity) in it, and lo and behold, it fell apart in my hands, revealing the interior where the chickadees had worked. Pretty impressive work for two small birds!