Propagation time of course;) This particular swarm, first tried a shallow cavity in the homeowner’s back yard tree 16 feet up from the ground to call there new home. I suppose the scout bees convinced everyone that is was “big enough” and got overruled on the cavity’s size to house everyone once they arrived? A day or so later (never wait that long to call a bee keeper), they rejected the tree cavity, and came down to settle on a lilac bush. The longer one waits to collect a swarm, the more likelihood of the scouts bees/swarm choosing a not so idea home, in your home, or large tree cavity in your yard. Obviously, most homeowners would prefer the tree cavity any day;)