Allbirds Performance Quarter Zip Purple Hush Merino Wool The Performance Quarter Zip is the thickest top we tested in this group and provides the best insulation as a standalone layer, as we wore it comfortably during temps in the mid-30s. It’s also very generously cut, so size down if you are between sizes. True to the Allbirds ethos, the shirt is made from sustainable materials, including merino wool and wood-based Tencel fabric that has exceptional softness and enhances moisture-wicking and drying speed. The quarter zip sits just below the chin when fully closed and thumb holes are great for securing the sleeves under a pair of gloves or just tucking in your bare fingers.