The birds are "cache"ing in.

I found a chunk of apple tree root in the wood pile last fall with a sunflower seed stuck in it. I put the piece up and under the mud oven roof. Over the winter a few more seeds were stuck in the chunk. I have a few more pieces to slab up, and I think I will hang them various places around the yard.

 

Nuthatches like to look in the bark crevices for bugs to eat. They may have put the seeds there.