A bee question

I have a bee question (tee bee?). I've read numerous stories on the phenomenom of bee colonies collapsing. I've also begun to read stories warning of the possible disaster looming if there are not enough honey bees to pollinate crops. Here's my question, though: How do we know the bees are dying? Almost all of the informational stories talk as though the bees are dying, but they also all have a line in them that say the bees are disappearing from the colonies with no bodies to be found. Maybe the bees aren't dying but moving on.

My father got me on this thought process. A couple of weeks ago we were talking over beers, and we were discussing the birds that visit his back yard. Last year they saw a number of birds in their backyard that they normally don't see in the city. The reason they saw many of these birds is because it was abnormally dry in their area last year, and fields out in the country became very hard. A number of birds moved into the well watered city to look for food. That led into a discussion of the honey bees that have took refuge in the corner of his porch last summer. I've encouraged their swift extermination, but he seems to have chosen to let them live there. We discussed the collapse of colonies and the possibility of that for his house hive, and he voiced an interesting thought. Maybe these colonies are not dying off, but perhaps there is something about their exisiting homes that is no longer beneficial, leading them to leave and form new colonies in more advantagous locations, much like the birds in his area adjusted their preferred locales when the summer weather made their existing ones harder to survive in.

I'll admit that I'm a bee simpleton. Maybe it is just not possible for this to occur, but I'd like to know if it is. If so, then commercial bee keepers do still have to worry, and the causation for the collapses still needs to be uncovered. However, if it is possible that these bees are relocating, than there really isn't anything to worry about crop-wise.