I would just leave the heads where they are and the birds will figure it out as they do every year when they forage. If you want to harvest the seeds yourself to put into a bird feeder, wait until the entire sunflower fades and browns. Watch the sunflower head as the seeds will grow and become more prominent. If they start to pop out, or if birds become interested, cut the head off and put inside a brown paper bag. Rub over the seeds with your hands until the seeds fall out and into the bag. Keep them in paper bags until next spring! I grew some great, big Mammoth Grey sunflowers from www.edenbrothers.com and they grew to be over 9 feet tall and I harvest the seeds from them every year!