Prior to the late 1990's, agricultural pursuits had dammed one end of Lake Apopka, in E. Central Florida for their aerial spraying operations (creating an air strip). When permits expired and were not renewed, facilities were demolished and the lake reflooded by removal of the dam. Following that flooding, the ground leached spilled and unreported chemical pesticides into the lake water at concentrations that killed a huge number of wading birds, white and brown pelicans, ducks, and shore birds/slow-wing-beaters (from duck hunting parlance). The sky for 360 degrees around the lake was dropping birds at alarming rates.
So, if this latest was a contaminated water source, birds in all directions and of all species would be directly affected. I am betting on an aerial poisoning [shootall is on to something with Crop Duster], as Black Birds tend to flock together and fly, en-mass, toward a common destination making them likely to fly through some concentration of toxicant that took them down.
