Welcome to the board, Tim! A timely query. Here in Cape May, New Jersey, we are descended upon every Fall by swarms of migrating monarch butterflies (we always called them swarms), they love the nectar of a certain kind of goldenrod that flourishes on the dunes and sandy soil here (along with milkweed), and it's always an inspiring sight to see the beauty of thousands of orange monarchs draped over the yellow blossoms along the beach! We always said "swarms" here...but I like "flutter" the best of all the suggestions posted here! Migration clusters...hmmm, "cluster"...we use that sometimes, too...can number up to the millions, I think, on their way to the mountains of Mexico and a certain new wintering spot in Florida (though not large enough to sustain the monarchs if they continue to lose their Mexican habitat). I have the figures somewhere...I'll post them when I can be more exact. I attended a Monarch seminar last year at The Wetlands Institute here given by a scientist who is the foremost expert/researcher on Monarch butterflies, so I'm sure the proper term is somewhere in the literature I filed from it. I'll look and get back if I find it.
