Another shot.

And one more. This one needs to be rotated 90 degrees clockwise.

It's a thistle.
It's likely from the birdseed.
After it blooms, the seeds will spread on the breeze, stuck to clothing, animals and more.
Remove it *now* unless you want a "crop" of the things.

Thanks jean001a! That makes sense. I had a finch feeder with thistle as well as a regular bird feeder with sunflower seeds, millet, etc. Amazing that these monster plants grow from a TINY thistle seed. I just wanted verification that they weren't a desirable wildflower before I yanked them out. Now you gave it to me.

Will these flower? The areas on the end of the branches sure like they would flower. Or will they just form seed pods? I'll google and find out now that I know what they are.
Thanks again!

They'll make some kind of flower - that's where the seeds come from. Yes, the birds would come and eat *some* of the seeds, but a lot would drop and grow more of these plants. If you're curious about the flowers, no need to pull the plant out before you see them. If you don't want that crop of plants next year though, pull it before those flowers make seeds.

Thanks purpleinopp. I am pretty curious about the flowers because they have been looking like they;re ready to "do something" at the end of each branch but week after week it's just more green. I might leave one of the plants and pull out the rest. In searching for thistle I haven't found a picture yet that looks just like the plants I have. The ones I see on the web show either a pinkish-purple flower or yellow. I'll post back here if they ever flower.

It's starting to look like a patch I could enter and never be seen again between these and the giant sunflowers, English ivy, one tree of heaven, and butterfly bushes. I'm keeping the butterfly bushes and sunflowers, cutting back the ivy and cleaning up the rest.

Update: Found out these prickly babies are safflowers. I had some of that in my feeder once over the winter so it makes sense. Here's the link to the answer by the person who solved it: http://forums2.gardenweb.com/forums/load/namegal/msg0612081210627.html?26205

Here is a link that might be useful: Hey! They're saffowers!