Does anyone know when to havest sunflower heads? I have read to let them turn brown but if i do this then all the stupid birds will have ate all the seeds. All the petals have fallen or dried up and the back of the heads are starting to turn yellow and brown on the edges. Is there anything i can cover them with besides a paper bag since i use a sprinkler head on my garden. Would a chicken feed bag work or would this keep the head moist. The bags are polypropylene feed bags.