From Aubrey Burl's Prehistoric Avebury (p.129):

"Around 2750 BC in the heat of a late summer, ants flying in the sultry air, people assembled at a low terrace of chalk jutting into the plain of silts and gravels where the Swallowhead spring now rises. Here they would build one of the wonders of the world, the largest man-made mound in Europe."

In my opinion, finding winged ants in the turves doesn't necessarily mean they were out and flying at the time, however, I do think that it's a good pointer to a summer start on the hill.