While huge flocks of blackbirds are not uncommon, particularly in agricultural settings, I was startled in early October 2019 by more than 1500 blackbirds with one thing in common--yellow heads.
When I first saw this huge flock of blackbirds (a "murmuration" in birding terminology), I just assumed that they were our common red-winged blackbirds. The birds were flying away from me, so I took a few pics and watched them twist and turn like a school of fish evading a shark.
Then these 1600 birds turned into the light and gold lit up the sky. They had yellow heads. All of them. I had seen a dozen yellow-headed blackbirds together before. Now I was seeing a hundred times more than that, all at once. I was mesmerized.
When they finally perched on a pivot irrigation sprinkler, they covered the pipes in yellow.
From studying the pictures closely, it does not look like there were any other blackbirds mixed in with these yellow-headed ones. They also appear to be almost exclusively male, which, according to the Cornell Lab of Ornithology, is not unusual when these birds are migrating across the Panhandle in the fall.
I dutifully reported my discovery to eBird, to find that mine was the highest count of yellow-headed blackbirds in Potter County ever reported to eBird. Not a rarity, but certainly a fabulous spectacle for me on that warm October afternoon.