I Heard a Tap, Tap, Tapping Coming From Inside the Tree~

We have several dead trees on our property that we leave for the natural sake of the matter.  One such tree no longer has a top, but is hollow down the center.  I heard a tap, tap, tapping coming from within the trunk of the tree and positioned myself and my camera ready for what might come out if I whistled.  I began to softly whistle and sure enough, up came a Yellow-Shafted Flicker.  It never even noticed me and busied itself with gathering ants, which is it's main food source.  In fact, because they do eat ants, they are often times on the ground and with no alarm system from others, are sadly easy prey for hawks and cats~