A note from Gisela,
I have several PMLNT members asking me how to detect our Garland Purple Martin pre-migratory roost and think this link below would be of help.
When the page opens there today at 7:00AM sunrise time, a round dark spot is appearing at the roost location in Garland Shiloh road Cinemark movie theater parking lot.
Check out the radar at: http://radar.weather.gov/Conus/southplains_loop.php
Today it is still visible not as large as we have seen it (8 of August) but it is still there. So this means the roost has not gone yet. The dark spot will eventually fan out towards Lake Ray Hubbard and then dissipate while the Martins are fanning out to feed at the surrounding location. I hope this will be of help.