Tinctures are classically made with high proof alcohol. Typically you are only ingesting like a dropper full (for humans) so the alcohol is not a big deal. If you are uncomfortable using it on your chickens, use the vegetable glycerin. It is basically a plant sugar.
I never gave my chickens tinctures, but I did run an herbal business where I formulated and made MANY tinctures using both alcohol and glycerin for human consumption. All the herbs on that list have well known benefits.
FYI: Alcohol is a more complete solvent. It more fully breaks down a wider variety of plant components. Also has longer shelf life. Glycerin's winning attribute is it is totally benign. It is thick and kind of hard to work with at room temp. It often requires double boiling low and slow heating with your herbs in it for a few hours to get a good extraction. I have never given my chickens tincture, or herbs for that matter. But I have taken lots myself! Just thought I would share some knowledge on tinctures in general. Don't be thrown off by the vodka if you want to do it. Alcohol is a tried and true solvent for extracting medicine from plants. But if your husbandry is all squared away, your chickens' respiratory systems should take care of themselves! No need for a preventative. If they are alreadhy sick, skip the herbs and get them veterinary medicine.
Tinctures are dope though. Make that one and use it yourself. We could all use a little respiratory strength these days.
Hope this helped a little.
