feral bees in black walnut tree

[wildwoodflowerfarm]

We have had feral bees living in a black walnut tree in our farmyard for several years now.  For the last 3 or 4 years the tree has been empty for varying periods of time and then we see bees again.  Then they will once more disappear.  The tree appears empty of bees at this time.  I do not see dead bees around the tree.  The bees are just suddenly gone.  I have seen swarms in the past but the tree does not empty at those times.  I have not seen a swarm lately.  Are the bees just going thru natural sycles or are they dying out because of some problem?  I would like to keep a colony of bees in the tree.  I like watching them.  Any suggestions about what is going on and what we should do?
Jere

[doak]

Feral bees will occupy a place for several years and then a bounden or abscond if you wish.
after wax moths etc. come in and clean it up another swarm may move in.
Once bees occupy a hollow or such it is almost impossible to keep them away completely.doak