In stage three, the barbs partially unite to form a central rachis, and even smaller filaments called barbules start to branch off from the barbs. This type of feather is also found in McKellar’s amber and some of them have barbules wit tight coils at their bases. Grebes – modern diving birds – have similar coils in their barbules. When they dive, the coils expand and allow the feathers to absorb water, making the grebe less buoyant and allowing it to sink. It looks like this adaptation was already around in the Cretaceous period.