The generally accepted wisdom that worms will migrate upward in search of food is so deeply ingrained in the vermiculture culture that I hesitate to challenge it, but not for long.
With my 10 years of off and on experience with the COW I have come to the following conclusions. Worms will migrate upwards if suits their convenience. Otherwise they will migrate down, up, sideways, diagonal, circular or in any geometric pattern that suits their whim. I suspect that they even migrate in and out of other dimensions. In the long run I think downwards, assisted by gravity is their first choice.
Today I decided to clean out a base station that had been in place for I don't know how long. It was not my first experience with this endeavor and it was typical. The station had a thick coating of black wet muck. In this muck was a very heavy concentration of worms. Far beyond the possibility of getting a count or even an estimate. I'll take a wild guess at 3000. They were in golf ball sized clumps throughout the entire area but especially, as I have noted before, in the grooves formed where the cylinders that enclose the legs and also provide ventilation. There are 5 of these with a groove on each side and each groove is packed with worm with very little soil. The drain spout is packed solid with worms. I wish that I had taken some pictures but even if I had I wouldn't know how to post them. I'm a computer retard. Happy worming, and remember, gravity is a worms best friend.