The Wisconsin birding list discussed this a few years ago. They documented similar behaviors with finches (esp. goldfinches) and thistle. They speculated that the seeds are being treated at such high temperatures (so they don't reseed) that they're drying out and/or that the long import time is drying the seeds out. Apparently, thistle seeds have a short shelf life.
I don't know if their explanation is right. But I'm adding this because birders in at least one other state are noticing the same patterns.
There is a juniper at the edge of my yard. Below it are all sorts of introduced plants that, I suspect, robins (and perhaps waxwings) have spread while visiting the juniper to eat her cones: a young Russian Olive, Poison Hemlock, and thistle. It's neat, though frustrating, to see how these plants cluster around a favored spot of songbirds. Meanwhile, I have several baby junipers growing around my yard, thanks, surely, to those very same songbirds.
- Jared Del Rosso