This is my third shampoo bar brand that I've tried. Lush was great, J.R.Liggett left a waxy residue in my hair. Yellow Bird is also leaving a waxy, sticky residue in my hair, it's so sticky it almost feels greasy. The instructions say to rinse with apple cider vinegar, which A. makes me smell like a salad, and B. is an acid, which makes me suspicious that the vinegar would be the only way to get rid of this residue. No thanks, I'd rather not smell like food.
I have fine (not thin!) hair. None of yellow bird's shampoo bars say whether they're good for fine hair, which is not the same as thin. In my experience, most people who have thick or curly hair have a great time with natural shampoos. My hair has never reacted well.
If you have fine hair, stay away!