For here again it is because the mouth fails to perform its office for birds have no teeth at all, nor any instrument whatsoever with which to bite up or grind down their food it is because of this, that in some of them what is called the crop precedes the stomach and does the work of the mouth; while in others the oesophagus is either broad or a part of it bulges just before it enters the stomach, so as to form a preparatory store house for the unreduced food; or the stomach itself has a protuberance in some part, or is strong and fleshy, so as to be able to store up the food for a considerable period and to concoct it, in spite of its not having been ground into a pulp.