how is the best way to store sunflower heads to use as bird food?

If you have a zip lock bag, big enough, put the head in that and seal it up, after it has dried out. The moisture will most likely cause some mildew to grow, that wouldn't be good for the birds. It may be easier to pick the seeds out and let them dry. The length of time may vary due to the size of the heads.

Harvesting---No more attention will be needed until the heads commence to ripen, when they should be looked to daily, as the seed soon falls if left too long and also, as the seed ripens, garden pests of the larger sort, birds and squirrels in particular, are always troublesome.

When the head shrivels and the seeds are ripe, cut the plants at the ground level, standing them with their heads uppermost, like shocks or sheaves of corn. When the heads are thoroughly dry, cut them off and thresh out the remaining seeds by standing each head on its side and hammering it with a mallet. Store the seeds in bags, in a dry place.

Hang them upsidedown in a cool dry place like a shed or garage. I've done this and it works nicely but if you leave them a long time put an old sheet or something underneath them because the seeds will eventually start to fall out.

cut the sunflowers and hang them upside down to dry and then rub the seed out and store them in a glass jar that is not packed tight for about a week in a dark place and shake them every few days them when they no longer feel damp you can package them into any thing that you want or you can dry them on the stalk and just put them out on the stem for the birds but hang them upside down to dry and stoer them that way

let them dry out, if you dont have the means then put them in an oven @ about 100 degrees for a few hours until dry keep an eye on it

pick out the seeds in the middle