Anything else you can tell us? Did the seed overwinter? How about a basal rosette of leaves? Any sap or scent when you break a leaf? There are features of the buds and upper leaves that are reminiscent of some thistles, but not with the relatively smooth younger leaves. The nyger thistle seed most commonly in bird seed combinations is not what produced this plant. This might not help you much, but maybe someone with more botanical knowledge than us will be prompted to reply.
