Of all the cases perhaps not every single one is tied together. In most cases it seems no one sees the birds die. No one sees sick or dying birds. Most towns just seem to wake up to find dead birds. Birds don't fly at night. Is someone silently trucking in dead birds? What are they laced with? The use of animal carcasses to spread disease goes back to one of the oldest forms of germ warfare. Using an animal carcass to refine a toxin is also a case of such. The animal will lay in the hot sun, swelling till finally it bursts, releasing it's toxins. Then again there could be no chemical cause.