African penguins have a black stripe curving across the top of the chest. They are insulated by air trapped between their feathers. This makes the birds extremely vulnerable when they are moulting, which they do annually, and for this period of about three weeks (at Boulders about November) they are land-bound, getting thinner and more bedraggled until the moulting process is completed. Before moulting they eat hugely and put on about 30% more fat. Moulting takes about three weeks, during which their weight almost halves.