By 8:00 AM, the flies had nearly all dispersed. I've seen similar aggregations at the exact same location in previous years. Over the next several weeks, I saw similar aggregations at this spot on sunny mornings.
With help from dipterists George "Jeff" Boettner at UMass, Amherst, and James O'Hara and Tarek Mohammed of the Canadian National Insect Collection ("CNC", Ottawa) the story of these flies can be told. Aggregations of male flies (and other creatures) like this are called "leks".
Jeff Boettner says, "Often I see this behavior on tops of mountains and such, and they will often use the same exact spot year after year. But yours is an amazing number of flies! Leks are basically bachelor hang outs, where boys play king of the hill and the boy holding the important spot when the female arrives gets to mate. They can often last just a few hours a day, so you have to know their schedule. But if you go back to the same tree in a year at the same time of day you may see it again. This a a great strategy for rare flies, (if you are rare it can be hard to find a mate, but if they have a preprogrammed spot- often a high point, or near water, and a preprogrammed time of day, it can limit the wasted time looking for a mate."
Subsequent to photographing these flies, I collected a sample and sent them to the CNC where Dr. Tarek Mohammed identified them and sent this kind reply: "Dear Dr. Malcolm, Your specimens have reached me safe. They are perfectly pinned and labelled. Such specimens are greatly appreciated by the Canadian National Collection of Insects. Thank you. The specimens are all males and belong to Pollenia labialis Robineau-Desvoidy, 1863. This species is distinguished from all other Pollenia species present in North America by the dark brown or black colour of the basicosta and posterior thoracic spiracle. Also, the facial carina is reduced or absent. Distribution: Northern United States and southeastern Canada. Pollenia labialis has been previously recorded from Ontario, Indiana, Michigan, Maine, New Hampshire, Oregon, and Washington. Therefore, these specimens are an important addition to the CNC. According to Rognes [1991: Blowflies (Diptera, Calliphoridae) of Fennoscandia and Denmark. Fauna Entomologica Scandinavica 24: 1-272], the biology of Pollenia labialis is unknown."
So, the Air Line Trail collection extends the known range of P. labialis into southern New England. A pity that more isn't know of this species' biology, but some other species of Pollenia are known to parasitize earthworms.