David,
Nothing to brag about, but I've seen my share of swarms over the past 25
years.  From my experience, it not uncommon to find a abundance of freshly
laid brood from a departing queen in the days (or daze) just prior to
swarming.  I have also seen, but less frequently, a lack of young brood in
the mother colony of a swarm.  When I attended a beekeeping course in France,
we were taught that the departing queen will lay a large number of eggs
before departing to assure the survival of the remaining colony.
Ed