It’s noon on a sunny Friday in San Francisco’s Jackson Square neighborhood and the Allbirds store, nestled in the shadow of the Transamerica Pyramid, is buzzing. Filled with tech bros and soccer moms, twentysomethings and retirees, most were returning customers, there to pick up a second or third pair of Runners, the now nearly ubiquitous monochrome wool sneakers, touted for their supreme comfort and selling for $95 a pair. In just three years, the aggressively normcore shoes have taken over San Francisco, to the point where this reporter spotted them on a hip designer dad, a pair of coffee shop brogrammers and a mom-of-three allergist all in one day. But the store wall displayed something brand new: a line of ballet flats, Allbirds’ first distinctly feminine silhouette. The flat, named the Tree Breezer, is constructed from an extremely light eucalyptus fiber and has the same merino wool insole and bottom sole (made from a carbon-negative sugarcane substance) as Allbirds’ other styles. It currently comes in four colors (with additional hues in the pipeline): navy, gray, and the brighter pink and turquoise. “This feels like I’m wearing a sock,” one young blond woman exclaimed as ...