The combine, late October.
Turns out the first swarm of grackles I saw back in September was a fluke. I haven't seen another all month, despite their being plenty of leftover corn in the field. I could easily collect a 55 gallon drum of full corn cobs in a few hours of picking. (I do pick up the occasional cob as supplemental sheep feed.) Last year the megaflocks were a regular occurrence in November and December. But then again we had a harvested sunflower field last year which is essentially a gigantic birdfeeder. Corn isn't as choice to a blackbird, I'd imagine. But it is to snow geese, which should be showing up en masse next month if last year is any guide.