[Ill.u.s.tration: FIG. 144.--Oriole's nest with skeleton of blue jay suspended from it; the blue jay probably came to the nest to eat the eggs, became entangled in the strings composing the nest, and died by hanging. (Photograph by S. J. Hunter.)]

Varying even more than the manner and power of flight among different birds are the vocal utterances, the cries and calls and singing. By their calls and songs alone many birds may be identified although they remain unseen. The field-student of birds comes to know them by their songs; knows what birds they are; knows what they are doing or not doing; knows what time in their life-season it is, whether they are mating, or brooding, or preparing to migrate; knows whether they are frightened, or self-confident, whether in distress or happy. Little urging and suggestion are needed to induce the student to attend to the songs. But the naturalist should not only hear and enjoy them, but by observation and the recording of repeated observations, he should come to understand the significance of the calls and songs.

As to how these sounds are made, attention has already been called (see p. 338) to the voice-organ or syrinx. The condition of this organ varies much in birds, as would be expected from the differing character of vocal utterances. Dissections will make these differences apparent.

=Nesting and care of young.=--Among the birds' most interesting instincts and habits are those domestic ones which include mating, nest-building, and care of the young. Birds' eggs and birds' nests are always attractive objects of search and collection for boys, and most boys have a considerable personal knowledge of the domestic habits of the commoner summer birds of their region. With this interest and unsystematized knowledge as a basis the teacher should be able to get from the cla.s.s much excellent field-work and personal observation. The first thing to undertake in this study is the gathering of data regarding the character of the nests of different species, their situation, the time of nesting, the partic.i.p.ation or non-partic.i.p.ation of the male in nest-building, etc.; also the number of eggs, their size and color markings, the length of incubation, the help or lack of help of the male in brooding, etc. In connection with this gathering of data in the field by note-taking, sketching, and photographing, nests and eggs can be collected (see directions on page 469). Let only one clutch of eggs of each species be taken for the common high-school collection, and if more than one nest is desired take used and deserted nests. When the nestlings are hatched, the bringing of food, the defence of the home, and the teaching of the young to fly should all be observed and noted.

Some attempt should be made to systematize the miscellaneous data obtained. Do all the members of a group have similar nesting habits?

Note the early nesting of birds of prey; note the nests of the woodp.e.c.k.e.rs in holes in trees; note the nesting of the various swallows. Is there any significance in the colors and markings of eggs? Observe the protective coloration obvious in some (see Chap.

x.x.xI). Are there differences in the condition of the newly hatched nestlings? Note the helpless altricial young of the robin; the independent precocial young of the quail.

The strong influence of the mating pa.s.sion will be made plain by observations on the fighting, love-making, singing, and general behavior of the birds in the mating season. The expression of the mental and emotional traits, the psychic phenomena of birds, are most emphasized at this time, and reveal the possession among animals lower than man of many characteristics which are too commonly ascribed as the exclusive attributes of the human species.

[Ill.u.s.tration: FIG. 145.--Western robin, _Merula migratoria propinqua_. (Photograph from life by Eliz. and Jos. Grinnell.)]

=Local distribution and migration.=--As explained in Chapter x.x.xII, the geographical distribution of animals is a subject of much importance, and offers good opportunities in its more local features for student field-work. The field-study of the birds of a given locality will comprise much observation bearing directly on zoogeography or the distribution of animals. Certain birds will be found to be limited to certain parts of even a small region, the swimmers will be found in ponds and streams and the long-legged sh.o.r.e-birds on the pond- or stream-banks, or in the marshes and wet meadows, although a few like the upland plover, curlews, and G.o.dwits are common on the dry upland pastures. Distinguish the ground-birds from the birds of the shrubs and hedge-rows and these again from the strictly forest-birds. Find the special haunts of swallows and kingfishers. Which are the shy birds driven constantly deeper into the wild places or being exterminated by the advance of man; which birds do not retreat but even find an advantage in man's seizure of the land, obtaining food from his fields and gardens?

Make a map on large scale of the locality of the school, showing on it the topographic features of the region, such as streams, ponds, marshes, hills, woods, springs, wild pastures, etc., also roads and paths, and such landmarks as schoolhouses, county churches, etc. On this map indicate the local distribution of the birds, as determined by the data gradually gathered; mark favorite nesting-places of various species, roosting-places of crows and blackbirds, feeding-places, and bathing- and drinking-places of certain kinds, the exact spots of finding rare visitants, rare nests, etc., etc. The making of such a zoogeographical map will be a source of great interest and profit to the students.

As already mentioned, many of the birds of a locality are "migrants,"

that is, they breed farther north, but spend the winter in more southern lat.i.tudes. These migrants pa.s.s through the locality twice each year, going north in the spring and south in the autumn. They are much more likely to be observed during the spring migration than in the fall, as the flight south is usually more hurried. The observation of the migration of birds is very interesting, and much can be done by beginning students. Notes should be made recording the first time each spring a migrating species is seen, the time when it is most abundant and the last time it is seen the same spring. Similar records should be made showing the movements of the birds in the fall. A series of such records covering a few years will show which are the earliest species to appear, which the later, and which the last. Such records of appearance and disappearance should also be kept for the summer residents, those birds that come from the South in the spring, breed in the locality, and then depart for the South again in the autumn.

Notes on the kinds of days, as stormy, clear, cold, warm, etc., on which the migration seems to be most active; on the greater prevalence of migratory flights by day or by night; on the height from the earth at which the migrants fly, etc., are all worth while. The Division of Biological Survey, U. S. Department of Agriculture, keeps records of notes on migration sent in by voluntary observers and furnishes blanks to be filled out by each observer. A suggestive book about migration, and one giving the records for many species at many points in the Mississippi valley is Cooke's "Bird Migration in the Mississippi Valley." Migration is discussed in most bird-books.

=Feeding habits, economics, and protection of birds.=--The feeding habits of birds are not only interesting, but their determination decides the economic relation of birds to man, that is, whether a particular bird species is harmful or beneficial to man. Casual observation shows that birds eat worms, grains, seeds, fruits, insects. A single species often is both fruit-eating and insect-eating. Do fruits or do insects compose the chief food-supply of the species? To determine this more than casual observation is necessary. The birds must be watched when feeding at different seasons. The most effective way of determining the kind of food which the bird takes is to examine the stomachs of many individuals taken at various times and localities. Much work of this kind has been done, especially by the investigators connected with the Division of Biological Survey of the U. S. Department of Agriculture, and pamphlets giving the results of these investigations can be had from the Division. It has been distinctly shown that a great majority of birds are chiefly beneficial to man by eating noxious insects and the seeds of weeds. Many birds commonly reputed to be harmful, and for that reason shot by farmers and fruit-growers, have been proved to do much more good than harm. Some few birds have been proved to be, on the whole, harmful. An investigation of the food habits of the crow, a bird of ill-repute among farmers, based on an examination of 909 stomachs shows that about 29 per cent of the food for the year consists of grain, of which corn const.i.tutes something more than 21 per cent, the greatest quant.i.ty being eaten in the three winter months. All of this must be either waste grain picked up in fields and roads, or corn stolen from cribs and shocks. May, the month of sprouting corn, shows a slight increase over the other spring and summer months. On the other hand the loss of grain is offset by the destruction of insects. These const.i.tute more than 23 per cent of the crow's yearly diet, and the larger part of them are noxious. The remainder of the crow's food consists of wild fruit, seeds and various animal substances which may on the whole be considered neutral.

[Ill.u.s.tration: FIG. 146.--Sickle-billed thrasher, _Harporhynchus redevivus_. (Photograph from life by Eliz. and Jos. Grinnell.)]

The slaughter of birds for millinery purposes has become so fearful and apparent in recent years that a strong movement for their protection has been inaugurated. Rapacious egg-collecting, legislation against birds wrongly thought to be harmful to grains and fruit, and the selfish wholesale killing of birds by professional and amateur hunters, help in the work of destruction. Apart from the brutality of such slaughter, and the extermination of the most beautiful and enjoyable of our animal companions, this destruction[18] works strongly against our material interests. Birds are the natural enemies of insect pests, and the destroying of the birds means the rapid increase and spread, and the enhanced destructive power of the pests.

It is a.s.serted by investigators that during the past fifteen years the number of our common song-birds has been reduced to one-fourth. At the present rate, says one author, extermination of many species will occur during the lives of most of us. Already the pa.s.senger-pigeon and Carolina paroquet, only a few years ago abundant, are practically exterminated. Protect the birds!

FOOTNOTE:

[18] One of the most unfortunate and conspicuous examples of this slaughter is the partial extermination of the song-birds of j.a.pan in the interests of European milliners. To meet their demands the country people used birdlime throughout the woods with disastrous effectiveness, as shown in the present exceeding scarcity of birds and the abundance of insect pests.

CHAPTER XXVIII

BRANCH CHORDATA (_Continued_). CLa.s.s MAMMALIA: THE MAMMALS

THE MOUSE (_Mus musculus_)

TECHNICAL NOTE.--It is best to catch specimens alive in a good trap.

A live trap well baited and placed in some old granary should furnish plenty for cla.s.s use. White mice can often be obtained at "bird-stores." When mice are not procurable, use rats. A rat is perhaps preferable on account of its size, but all essential structures can readily be made out in the mouse. Specimens should be killed by chloroform as described for the toad, p. 5.

=Structure= (fig. 147).--Compare the external characters of the mouse with those of the toad and sparrow. The mouse, unlike the other vertebrates so far studied, is thickly covered with _hair_ all over its body except on the tip of the nose and the soles of the feet. Where are the _nostrils_ placed? What are the large leaf-like expansions called _pinnae_ situated just back of the eyes? Pull open the _mouth_ and note the large _incisor teeth_ on the upper and lower jaws. Cut one corner of the mouth back and observe the large flat-topped _molar teeth_ on both jaws. How does the attachment of the large fleshy _tongue_ differ from the condition in the toad? The toad's tongue is for snapping up insects, whereas in the mouse this organ serves to move food about in the mouth.

On the tongue are numerous small _taste-papillae_. Notice the long hairs, "feelers," on each side of the nose. Note the similarity between the front paws and our own hands; each has four fingers with a small rudimentary thumb on the inner side of the paw. How does the hind foot of the mouse differ from the foot of man? Posteriorly the body is terminated by a long _tail_. At the root of the tail is a small aperture, the _a.n.u.s_, and just below, or ventral to it, is the opening from the kidneys and reproductive organs.

TECHNICAL NOTE.--Place the mouse on its back in a dissecting-pan and cut through the skin from a.n.u.s to the lower jaw. Extend the legs, pin down each foot and pin out the cut edges of the skin. Now carefully cut forward through the body-wall from the a.n.a.l region and on through the breast-bones and ribs. Pin each side out.

Near the hindmost pair of _ribs_ note a sheet of muscles, the _diaphragm_, which extends across the body-cavity, dividing it into an anterior portion, the _thoracic cavity_, and a posterior, the _abdominal cavity_. What are the most conspicuous organs in the thoracic cavity?

Leading anteriorly to the mouth-cavity is a long tube, the _trachea_, composed of a series of cartilaginous parts of rings placed end to end.

Note at its anterior end the _glottis_ and _epiglottis_. Insert a blowpipe into the glottis and inflate the _lungs_, which will fill all the otherwise unfilled s.p.a.ce in the thoracic cavity. The abdominal cavity contains the _viscera_ suspended in a fold of the lining membrane, as in the other vertebrates studied. Note lying against the diaphragm a large, red, glandular structure, the _liver_. Separate the two large lobes of the liver and expose the opalescent _gall-bladder_.

By pa.s.sing a canula into this and ligaturing, the _cystic duct_ may be injected. Beneath the liver is a large loop-shaped expansion of the alimentary ca.n.a.l, the _stomach_. Arising from the right end of the stomach is the narrow _duodenum_, which gradually merges into the very much convoluted _small intestine_, or _ileum_, which is followed by the _large intestine_, or _colon_, the last part of which is a straight tube, the _r.e.c.t.u.m_. The small intestine occupies most of the s.p.a.ce in the peritoneal cavity. Within the loop of the pylorus will be found an irregular pinkish ma.s.s of tissue, the _pancreas_. Beneath the stomach on the left side of the body lies a very dark glandular ma.s.s not much unlike the liver but altogether detached from it. This structure is the _spleen_, a ductless gland.

Note dorsally of the trachea a long tube pa.s.sing through the diaphragm and connecting the mouth with the stomach. What is this tube? Note the _Eustachian tubes_ extending from the mouth to the ears. The median part of the roof of the mouth is the _palate_, hard in front, soft behind. A pair of small bodies at the sides of the soft palate near its hinder end are the _tonsils_. At the posterior angle of the lower jaw are glandular bodies, the _sub-maxillary glands_, which lead by a short _duct_ anteriorly to open on the floor of the mouth. On the sides of the neck just below the ears are pink or yellowish bodies, the _parotid glands_, opening anteriorly in the sides of the mouth-cavity. These two sets of glands are collectively known as the _salivary glands_, the function of which is to secrete the saliva.

Push apart the sub-maxillary glands and note below them overlying the trachea on either side two dark-red lobes connected by a band of tissue. These const.i.tute the _thyroid gland_, another of the so-called ductless glands. Within the thoracic cavity anterior to the heart note a ma.s.s of pinkish tissue, the _thymus gland_. Observe the large _ma.s.seter muscles_, which cover the jaws. What is their function? On either side of the neck lies a large blood-vessel, the _external jugular vein_, which collects blood from the head and carries it down to the heart. Note the large _pectoral muscles_ which cover the breast and extend out into the arms, and which are so strong and highly developed in the sparrow. The head is supported by large muscles which run down the back of the neck to the ribs. Others are attached to the ribs, which they raise and lower. These movements, together with the contraction of the diaphragm, cause the expansion and contraction of the thoracic cavity whereby the lungs are regularly filled and emptied. Note that the abdomen is covered by a double layer of muscular tissue, the outer part made up of the _external oblique muscles_, the inner by the _internal oblique muscles_.

[Ill.u.s.tration: FIG. 148.--Diagram of the circulation of the blood in a mammal; _a_, auricles; _l_, lung; _lv_, liver; _p_, portal vein bringing blood from the intestine; _v_, ventricles; the arrows show the direction of the current; the shaded vessels carry venous blood, the others arterial blood. (From Kingsley.)]

Examine the _heart_. How many _auricles_ has it? The _ventricles_ in the mouse, as in the bird, are entirely separated, forming two complete compartments, a _right_ and a _left ventricle_. The blood flowing from the veins of the body is collected in the right auricle, thence it pa.s.ses into the right ventricle, whence it is conveyed to the lungs; returning it flows through the left auricle into the left ventricle, whence it is forced through the arteries of the body. For a study of the circulatory system in mammals (fig. 148), a rat or a rabbit should be injected by the teacher and an advanced text-book, as Parker's "Zootomy"

or Marshall and Hurst's "Practical Zoology," used as a guide. A sheep's heart is very good to cut open for a cla.s.s demonstration.

Make a drawing of the organs observed thus far in the dissection.

The _kidneys_ in the mouse are situated in the dorsal region next to the backbone. They consist of two bean-shaped smooth glands. From them a pair of ducts, the _ureters_, can be traced down to a median thin-walled muscular sac, the _bladder_. The bladder opens to the exterior of the body by means of a short tube, the _urethra_. Cut open a kidney longitudinally and examine the cut surfaces.

The two egg-glands of the female mouse lie in the median portion of the abdominal cavity, somewhat below the kidneys, and from the vicinity of each runs an egg-tube. These tubes meet below the bladder, and open to the exterior of the body through the aperture noted below the a.n.u.s. In the posterior parts of these tubes lie until birth the developing embryos.

TECHNICAL NOTE.--For a study of the nervous system place the specimen ventral side down and cut through the skull with the bone-cutters or heavy scissors, exposing the brain and spinal cord.

Note the large _brain_ (fig. 149), composed of small _optic lobes_, large _cerebrum_, _cerebellum_, and _medulla oblongata_, followed by the long _spinal cord_. Note the nerves arising from the brain and spinal cord.

[Ill.u.s.tration: FIG. 149.--Diagram of brains of vertebrates; _Olf. L._, olfactory lobes; _Cbr._, cerebrum; _Md. Br._, midbrain (optic lobes); _Cbl._, cerebellum; _Med. Ob._, medulla oblongata; _Sp. Cd._, spinal cord. (From specimens.)]

For a careful dissection of the mammalian nervous system a larger mammal, such as a cat or dog or rabbit, should be used. For guide use a text-book such as, for the dog, Howell's "Dissection of the Dog"; for the cat, Reighard and Jennings' "Anatomy of the Cat"; and for the rabbit, Parker's "Zootomy" or Marshall and Hurst's "Practical Zoology." Make a good preparation of the brain and preserve it for future use in some fluid like Fischer's fluid (see page 453).

TECHNICAL NOTE.--Prepare a well-cleaned skeleton by boiling a specimen in a soap solution and thoroughly cleansing it (see p.

452).

Note the very compact _skeleton_ of the mouse. Note the closely sutured _skull_. How many _cervical_ or _neck vertebrae_ are there? The _ribs_ are attached to the _thoracic vertebrae_. How many pairs of ribs? The bony thorax supports the _shoulder-girdle_ and bones of the fore legs.

The thorax is followed by a series of ribless vertebrae, the _lumbar vertebrae_, which in the posterior region of the body fuse with the _pelvic girdle_ supporting the hind limbs. The body vertebrae are succeeded by the very much smaller _caudal vertebrae_. Compare the skeleton of the mouse with that of the bird; also with that of the toad.

For directions for a detailed study of the skeleton see in Parker's "Zootomy" an account of the skeleton of the rabbit, pp. 263-286.

TECHNICAL NOTE.--For the study of the eye (fig. 150) the teacher should obtain the eye of some large mammal, as the ox or sheep, with which to make a cla.s.s demonstration. The eye of a rabbit or cat can of course be used. For an account of the vertebrate eye see Parker and Haswell's "Text-book of Zoology," Vol. II. pp.

103-107. For a study of the ear use a bird or mammal, and see pp.

107-110 of the same book.

[Ill.u.s.tration: FIG. 150.--Diagram of vertebrate eye; _c_, choroid; _i_, iris; _l_, lens; _n_, optic nerve; _r_, retina; _s_, sclerotic.

(From Kingsley.)]

=Life-history and habits.=--The house-mouse is not a native of North America, but was introduced into this country from Europe, to which, in turn, it came from Asia, its original habitat. The mouse came to this country in the vessels of early explorers. Similarly the brown and black rats, now so abundant all over North America, and members of the same genus as the mouse, were introduced from Europe. Accompanying man in his travels the mouse has spread from Asia until it is now to be found over the whole world.

The habits of mice are well known; their fondness for living in our homes and outbuildings makes them familiar acquaintances. Their food is varied; they seem to thrive best, however, on a vegetable diet.

Grains and nuts are favorite foods. The house-cat is their greatest enemy, but man takes advantage of their instinct to go into holes by constructing traps with funnel or tunnel entrances which, baited with cheese or other favorite food, are fatally attractive. In climbing, mice are aided by the tail. Their strong hind legs enable them to stand erect, and even to take several steps in this posture. They can swim readily, although naturally they rarely take to water. Their special senses are keen, the senses of hearing and taste being unusually well developed. Their "singing," which has been the subject of much discussion, seems to be actually a voluntary and normal performance which, however, hardly deserves to be called singing, but rather a slightly varied peeping or whistling.

The mouse is a prolific mammal, producing from four to six times a year broods of from four to eight young. The mouse makes a cosy nest of straw, bits of paper, feathers, wool or other soft materials, and in this the young are born. The newly born mice are very small and are blind and helpless. They are odd little creatures, being naked and almost transparent. They grow rapidly, being covered with hair in a week, although not opening their eyes for about two weeks. A day or two after their eyes are open they begin to leave the nest, and hunt for food for themselves.