We had an influx of Hoary Redpolls this year - a first for me, and for the yard! Pretty neat! There was only about a dozen total, including a few Commons, but the Hoaries seemed to outnumber them. The thing is that there didn't seem (to me) to be a real sharp division between Common and Hoary.
When they're in front of you, there is a noticeable difference. Some look white. Frosted. Hoary, if you will. The Commons can look quite pale, but it's always light grey, or light tan. The Hoaries SHINE! But it doesn't always translate into photos. As well, there seemed to be a sliding scale for flank streaking. Some of the birds had almost none. Some had more than one might expect from a Hoary. I'm not sure how to interpret that. I think some folks would say those streaky birds aren't definitely Hoaries. But when they're in front of you, man, do they look different from the Commons, of which I've seen a LOT!
But call them intermediates if you like.
I didn't have a lot of luck getting photos of undertail coverts, but had better luck with rump shots. So for your viewing pleasure, I present a variety of Hoary Redpoll rumps.
Birders are awesome. Who else would see nothing untoward in a collection of bird butts?