Bob Miller went out to the NRW Imperial site near Imperial, Imperial Co. CA on the evening of Dec. 4, 2001 and was totally blown away by the numbers of Rambur's Forktails!! He estimated 10,000 as a conservative number. They were literally lining the sedges. He saw 11 of the orange females and was only able to find about 15 other females. They were almost entirely males! On previous outings to this site in days just prior to this 'event' he usually found ~ dozens of I. ramburii, so these #s seem quite high. No other dragonflies were seen but he was there very late and the pictures (links follow) were taken after dark by flashlight &/or flash.
Links to images:
hoards of I. ramburii
young female I. ramburii by flashlight
many I. ramburii
many more I. ramburii
more I. ramburii - being stalked by a spider!?
Doug Aguillard of San Diego was at the same site three days later on the 6th, and there were only hundreds of the Rambur's left at the location.
Bob Miller then went back to the Imperial site again the evening of Dec. 13, 2001 and to the nearby Brawley site the morning before and searched valley wide for two days but found NO dragonflies or Damselflies anywhere!! So, as he put it, one heck of a lot of I. ramburii from just one week prior were just gone! He even crawled around looking on the ground beneath the sedges looking for corpses......none!
He wrote that the last night that he saw them there was a front coming in and the strong wind was quite cold. He had thought they would be down low in the vegetation but to his surprise they were crowding the tips of the highest Sedges (see links). The weather was pretty much cold and stormy for the next few days after that, although there was no rain and it was about 50% cloud and sun. He wrote that the temperature was probably mid 30s for the low but a little cooler in the river bottom. This is an agricultural area so ground level temps for lettuce ice in the fields are broadcast and there were no such broadcasts.
He has seen no I. ramburii since that date, although he has seen a Variegated Meadowhawk & a few Roseate Skimmers.
I guess I'm wondering, did they just die, or were they preparing to take off and migrate?? And why so many at that late date?