Birkenstock Kay Clogs. The BIRKENSTOCK Kay is a classic clog featuring a convertible backstrap with a buckle closure that can swivel back for a more secure fit. The upper is made from the skin-friendly, hard-wearing synthetic material Birkibuc®. Anatomically shaped cork-latex footbed. Birkibuc® upper. Suede footbed lining. EVA sole. Two flexible straps, each with an individually adjustable metal tongue buckle. Imported.