The annual gathering of Purple Martins beneath the Causeway is underway! I visited the roost at the Metairie shoreline this evening, and could detect a couple thousand birds. Keep in mind that, when I studied the roost extensively three years ago, counts of the departing birds in the morning (a steady directional flow that can be quantified easily) were consistently much higher than could be obtained in evening (when they form swirling masses). As usual, they were clinging to the edges of the lengthwise concrete beams underneath the bridge, both on its west and east faces and underneath. My poor and grainy video shows the scene on the west face.
The appeal of the Metairie roost is that some of the birds are quite close, and can be easily viewed while perched. As I have noticed before, the birds appear to be essentially all females and immatures- which have pale gray on the underparts and much of the head. Out of a couple hundred examined closely, none were adult males (which would appear all black in dusk lighting).
The roost at the Mandeville end of the Causeway is usually much larger, but is too far from shore to be seen from land. Three years ago when I kayaked out at night (not advisable- lots of speed boats!) I was able to reach 125,000 of them and estimated that the total number may have been twice that, based on the extent of the whitewash seen there from a boat during the day..
That roost off Mandeville is active again this year, as judged from the green blob in last night's radar image (thank you, rap.ucar.edu):