I have always had goldfinches in my garden. They come to the feeder for the niger thistle seeds, and they eat seeds of the conflowers, tithonias, and smaller headed native sunflowers and sunchokes in the garden (they also eat mustard family seeds -- radish, kale, etc. -- and shiso seeds). But I have never seen this particular behavior before, though I admit that I may have somehow missed it all this time, and they have been doing it all along....
Day before yesterday and yesterday, I plucked off unopened dandelion seed heads (I used the stems as mulch) and put them in the tube feeder because I saw someone post that they save and feed dandelion seeds to the birds. I thought "why not?" since I don't mind them growing in my lawn, etc. if they end up being scattered off of the bird feeder.
Yesterday when I added a double handful of the seed heads, I saw that the ones I added the day before had opened inside the tube.
I'm wondering if they learned that dandelion seeds are good to eat because they ate them at the feeder.
Right now, they are in my "Sunflower House" bed of corn, peas, fava beans and cilantro, in the interior rows that I can't access right now because of the heavy overnight rain, searching out all the dandelions that have gone to seed... 
BTW, while I was watching the goldfinches, I saw an unusual visitor -- an Indigo Bunting
Does anyone know if Indigo Buntings hang out with Goldfinches? This is the second time I've seen Indigo Bunting while I was noticing a flock/harem of goldfinches arrive and do something in my garden. ...or maybe it's just a coincidence. 
