Sunday, November 24, 2013
mega Tree Swallow roosts on internet radar
One of the more spectacular late fall events in the birding calendar of southeast Lousiana is the formation of huge Tree Swallow roosts. These are usually in sugar cane fields, and usually persist until the cane is cut around the present date.
This year there have been no (publicized) reports of such roosts, including from the usual stronghold, a location on the West Bank upriver in Vacherie. The birds normally become a milling mass that covers the sky near twilight over the roost field, and eventually settle down into the cane through a weird downward extension of the flock that looks like a funnel cloud- birders often refer to it as a "swallow tornado." This conduit drains the great mass of swallows from the sky until they are down in the cane for the night. It is quite possible that the Vacherie roost has a million birds, but estimating is very difficult.
I noticed yesterday morning that two roosts appear to be visible on online weather radar (unfiltered, thus the birds remain visible; such images are available at rap.ucar.edu). It is normal to be able to see the dispersing roosts in dawn images on such radar. Although other densely roosting species can cause similar radar echoes, Tree Swallows are the most likely this time of year (and Purple Martins in summer; possibly other species at other seasons also).
Interestingly, the present roosts seem to be in wetlands- not cane. One is north of Thibodaux (about one mile south of Hwy 3219 x Hwy 3127), and the other appears to be near the Airline Hwy entrance to the Ormond Estates, west of the I-310 overpass! That is, in the wetlands north of Airline. There may be interesting viewing there in the evening- anyone with any observations, please let me know!
Here are two images from yesterday near dawn. The red arrow in the top image shows the Ormond roost beginning to disperse (small dot). The second image shows both roosts dispersing shortly thereafter- the Ormond birds have now created a crescent echo headed east or northeast; the red stars are where the two echoes first appeared, and are the best estimates of the actual roost sites. There was some hint of another roost at Manchac, which may have merged with the Ormond birds in this second image. The fact that the radar echoes are moving east is probably just an artifact of the position of the radar beam coming from Slidell- they probably disperse in all directions.