Echinacea seeds

After first couple years of growing Echinacea purpurea (purple coneflower), I found that they like to spread modestly with volunteers. Since I moved every year back then, I started to cut down the seed heads by late fall so that I don't have to worry about finding homes for those little volunteers the following spring. One day in early fall last year, we found ourselves again in a home improvement store and Dan pointed out a farmer's almanac calender on display, which we brought home.
Glancing through the illustration and gardning tips in the calendar, I found that goldfinch *loves* Echinacea seeds, and I *love* goldfinch, so I let Echinacea plants go seed and didn't cut them down this fall. Goldfinch didn't wait till winter to eat those seeds. As soon as the seeds started to mature, I had little visitors flying in and out with yellow streaks. They picked one group of plants completely out of seeds but left the other group with some seeds. I wonder why.
I hope to have a nice patch of sun flowers this year. One summer Shannon had beautiful sun flowers in her back yard and goldpinches were hovering there forever. It was beautiful especially when the Sun was low shedding golden rays everywhere.