An observer near Houston noticed what happened when north winds arrived in Texas last week.
"On the 25th we had a wind out of the northeast at 5-7 mph. Nearly all of the hummers took that wind south. The mass exit happened in a day and a half. I went from 75-100 hummers down to 5-7. September 26, 2015 Scott Brega Huntsville, Texas
Winds on September 25, 2015