Birds moult or shed their feathers. The summer dress in many species varies from that of the winter.
The mode in which the plumage changes in well described in the 'Transactions of the Zoological Society' by Yarrell; and the same able zoologist has shown in the 'Philosophical Truneactions,' and in the ' Proccodinga of the Zoological Society,' that the putting forth of the plumage of the male bird is not confined to the female put the age of reproduction (so many well-known instances of which are given by Dr. Butter, John Hunter, and others), but that the garb of the cock in assumed by those hen birds which from malformation or disease arc rendered unable to assist in the continuation of the species.
The following three modes by which changes in the appearance of the plumage of birds are produced have been pointed out by Yarrell : 1, By the feather itself becoming altered in colour. 2, By the bird's obtaining a certain portion of new feathers without shedding any of the old ones. 3, By an entire or partial moult, in which the old feathers are thrown off and new ones produced in their places. The first two of these changes are observed generally in the spring, indi cating the approach of the breeding season ; the third is usually partial in the spring, and entire in the autumn. The subjoined cut is explanatory of the situation of the principal parts of the plumage, particularly those most conducive to flight.
That the skin and integumente of birds perform the office of emunctory organs appears not only by their moulting, but also by the quantity of mealy dust separated from the skin in many birds. The cockatoo, for instance, discharges a quantity of white mealy dust from its akin, particularly at pairing time, according to Blumenbach ; and Bruce, in the appendix to his ' Travels,' gives an account of his shooting a large bearded eagle, which, on his taking it in his hands, covered him with a powder which was yellow on the breast, where the feathers were of that colour, and brown on the back, where the plumage was of the same hue. A heron too which he shot is described as having a great quantity of blue powder on the breast and back.
The glands which secrete the oil used by birds in preening and dressing their plumage are situated on the upper part of tho taiL Water-birds necessarily require a larger portion of this protecting fluid, and accordingly we find the glands largest in that race. Wenn:um observes, that in that variety of the common fowl which has no tail (Gallus ecaudatus) these glands are absent.
Digestive Organs.
The bill has a horny covering which in some degree answers the purpose of teeth, and indeed it is in many instances notched so as to represent them. The form of this important organ varies greatly,
but with evidence of the most perfect design in each varied instance, according to the nature of the necessary food. Thus in birds of prey it well executes the office of a dissecting-knife ; in seed-eating birds it forma a pair of seed-crackers for extricating the kernel from the husk which envelops it ; in the swallows and goatsuckers it is a fly-trap; in the swans, geese, and ducks it is a flattened strainer, well furnished with nerves in the inside for tho detection of the food remaining after the water is strained by that particular operation which every one must have observed a common duck perform with its bill in muddy water. In the storks and herons we find it a fish-spear; and in the snipes and their allies it becomes a sensitive probe, admirably adapted for penetrating boggy ground, and giving notice of the presence of the latent wornt or animacule. The food is transmitted from the bill through the cesophagus into the stomach, which is composed of three parts, namely, the crop, which is a dilatation of the cesophagus, and ties just before the breast-bone; the membranous stomach (ventricule suceenturi6 of the French) ; and the gizzard. The first of these is furnished with many mucous and salivary glands ; in the next (and the structure of this may be best observed in the gallinaceous birds) there are a number of glandular bodies which pour out a copious secretion to mingle with the food as it is ground down by the powerful gizzard, which reaches its highest development in graminivorous birds. This mill is rendered still more effective by the swallowing of small hard stones by those birds with their food, a practice which is clearly instinctive, and carried sometimes to a great extent. In the museum of the College' of Surgeons (London) is a large glass bottle entirely filled with pebbles, &c. taken from the stomach of an ostrich. The well known experiments of conveying bullets beset with needles and even lancets into the stomachs of graminivorous birds, with the effect of the total destruction of those sharp instruments in a short period, need only be referred to here ; but as Felix Plater's observations have not attained quite eo much celebrity we shall shortly mention them. He found that an onyx swallowed by a hen was diminished one-fourth in four days, and that a louis-d'or lost in this way sixteen grains of its weight.