Hi Donald, and others,
Debi is right, there is a shortage of bees! So keep that swarm in your hollow oak tree as happy as possible! (That might mean to be careful using pesticides in the garden when things are in bloom, too.)
I have kept bees for eleven years. My father-in-law is also a beekeeper with 300+ hives (I only keep 3 to 5 beehives on our dairy farm.)
Without knowing when the last rain was in your area, or if there are any pollen or nectar-producing plants in bloom in Texas right now, I would have to say that they are trying to get moisture out of your pots.
Every spring the first thing my honeybees fly to when the weather is above 60, is the woodpile. They literally ROLL around in the sawdust shavings -- rubbing it all over themselves -- and fly back to the hive. There must be trace amounts of sap left in the sawdust, especially with moist ground underneath it in the springtime, and the honeybees need that semi-preserved nutrient to build up a colony before the first nectar-flow comes from local wildflowers or trees.
I can definitely guarantee you that they are NOT swarming. Flying swarms look like a dark cloud in the air. A swarm that has landed is so dense you cannot possibly see every individual bee. Swarming only happens when a colony is moving from an old location to another, or if they are dividing into two colonies. One group of bees re-locates with the old queen leaving behind a new queen with the other group of bees to keep on "business." This is the ONLY way that honeybees naturally and externally (out-of-the-hive) "procreate" -- to expand the pollination area that keeps nature going. Otherwise, without human "help" they would all be in one region of the world.
To test and see if it's water they want, then water only ONE of the pots in your section, and see which one they hover over more.
If you want to have a little fun, later on put some sugar in that water and water a different pot. THEN see which one they like.
But --- Keep your mouth closed! :-)
President - Mio Irisarians
Zone 4 (and a half . . .J)