The roosts are impressive at dusk, when huge clouds of swallows cover the sky. Their roost entry is very strange, as the birds do not descend into the cane over a broad area, but instead "drain" from the cloud through a small funnel-shaped pathway they create. For this reason, this phenomenon is often referred to by local birders as a "swallow tornado."
Cane fields are generally private property, so viewing options may be limited. Sometimes a nearby river levee provides the best viewing, but sometimes engaging a farmer in conversation can open a door to driving out into the heart of the event and standing next to the funnel.
The roosts are generally visible on the Slidell radar, when accessed online at a site that does not filter out birds. The image below is from rap.ucar.edu, taken this morning (the 12:29 UTC on its label indicates 6:30 local time). There are two obvious donut echos, one of which appears to be between Paincourtville and Belle Rose along (east of) Bayou Lafourche, and the other just east of Laplace, perhaps visible from Hwy 61. Hidden between them (partially covered by the Belle Rose echo- only the southeast edge of the donut is visible) is a third one, seemingly near Vaccherie or St. James.