Black eyed Susans
Swamp Milkweed
Orange Milkweed
Blue great lobelia
10 petal sunflowers
Lance leafed goldenrod
a native aster

All have gone to seed.

I don't want to cut them down because I want the birds to have access to the seeds in winter. But is now the time to separate them? Or in the spring? It's a bit crowded in there.

Any other advice?

I would personally wait until spring when they just begin to sprout and leaf out. Some may not survive the winter - Mother Nature weeds out the weak and unlucky. In the spring move around the survivors that need more space. Just mark your plants well now. The freezing and thawing and too often heaving of soil through the winter will push out tiny plant markers and by spring you won't remember what is where. (Yeah.....personal experience!)

FataMorgana

Around here, we sometimes get the "dustings" of snow - Very pretty. More often, though, we get enough to bury the seed heads completely. I personally prefer lots of snow!

I'm so glad you added that comment. I feel the same way. A blanket of snow covering a lawn is nothing in comparison to the beauty and interest of seedheads with frost or covered in snow.

Great advice from all, so I really don't have to add anything. I will, however, say that dividing in the fall is another option...especially when you know the plants will have enough time to put down roots before the ground freezes. From what I've read, the roots continue to grow well into the fall and early winter if the soil has not frozen to any significant depth.

That said, I think I tend to divide more often in spring than in fall.