> The point is, why would the bees need a mechanism to measure how much honey is in the hive if they have a mechanism to choose an appropriate sized cavity to begin with? 

Exactly!

> The strategy that seems to have evolved over time is that bees occupy a relatively small cavity, build up to fill the cavity early enough in the season to swarm (at least once).  This works because they tend to choose cavities of a size that makes this work.

Right, they do not pick the size on the basis of how much room there is for honey per se, but more likely on the basis of the average size that a colony needs for an average size nest, one that offers adequate protection from predators and the weather. 

> Now, why do beekeepers (who have been expanding colonies and trying to prevent swarming all season) have late swarms that have no chance of survival?

This is a major question, in my mind. Maybe they think it is spring, based on the day length (the day length in late August is the same as late April. (This is wild speculation on my part, something I can do as well as anyone).

New work presented by Wyatt Mangum has documented a previously unobserved phenomenon: late swarms often force their way into colonies, essentially getting their winter supplies for free. I have no idea how often this happens. 

PLB 

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