Ann, I think you're probably referring to the Monarch migration which seems to happen every September here, they all seem to gather or congregate, nectaring energy for the long journey south. They seem to zero in on a particular wildflower and then, one day (in Sept.), they're gone. I can recall the last few years, one year they seemed to mass on the New England Aster, another year it was Maximilian's Sunflower, a different year Bidens, one year goldenrod. Not to say all the monarchs went to only one wildflower each year as you could find some in other stands but they did seem to favor different (wildflower) stands here each year.
I've only seen a few Monarchs this year. I need to go to a near by state park where the Dept of Natural Resources preserve nature pretty much as nature meant to be. That's where the action is.
Anyway, here's the blues -
