Over the course of many years, my casual observations of swarming in my
home yard agree with Thom Bradley's experience in the following details:

1.  For east-facing hives, swarming tends to be directly down wind from
the prevailing westerly winds or at a ninety degree angle to the north
of the hives.

2.  The downwind swarms that I happen to witness always stop on a false
cypress (Chamaecyparis sp) about 50-60 yards (45-55 meters) east of the
hives.  They obligingly land on low branches and are a cinch to
retrieve.

3.  Northbound swarms that I (or my neighbors) happen to witness pause
either high in one of three white pines (difficult to recover) or at
mid-height in a juniper (easy to retrieve).

4.  Swarming seems to take place mid-day, roughly during the part of the
day that is preferred for most photography.

All observations have been accidental.  Beekeepers’ lore has it that the
surefire way to prevent swarming is to stay out of sight of the beeyard.

Joseph Cooper
Latitude: 40.083661; Longitude: -83.010478