Popular Science Monthly/Volume 30/January 1887/The White-Footed Mouse



OFTEN, as early in autumn as the first of October, the abandoned nests of cat-birds and cardinal grosbeaks, and to some extent those of the brown and song thrushes, will be found very frequently to be tenanted by those beautiful little mammals, the white-footed mice (Hesperomys leucopus). While the fact of such situations being chosen by these mice, for their winter quarters, has been long known, I am not aware that observation has been carried beyond this point; and I recently endeavored to determine, first, to what extent these old birds' nests are remodeled; and again, whether or not some of them may not be constructed de novo, the builders using the abandoned home of a bird for the exterior of the new structure, and removing it, bit by bit, from its original site.

In the months of October and November of the past year (1885) I examined a series of forty-two nests, all of which were above the ground, and occupied by mice. All were strikingly different from any nest of a bird, such as is found in so exposed a position; none being open above, nor having the materials for linings such as our thrushes and larger finches are accustomed to use.

Of the series thirty-one were placed in dense tangles of Smilax rotundifolia, or green-brier. None were near the upper or outer edges of the thicket, but usually about one third the distance from its uppermost surface, and midway from side to side: for instance, if the growth was ten feet high and six or eight in width, the home of the mouse would be at an elevation of between six and seven feet; and it had therefore a protecting growth of thorny smilax of three to four feet in extent above it, and nearly the same upon each side.

This was a very uniform feature of the series examined, and, if the mice merely occupy old nests of birds, indicates a uniformity in the matter of their locating by the birds, of which I was not aware, and which I am inclined to doubt.

Again, the smilax was so very dense or closely intertwined, in the majority of instances, that it was clearly impossible for a bird as large as a robin or grosbeak to have penetrated it with that celerity of movement necessary to escape the impetuous charge of a hawk. It is, I think, far more probable that the continuous growth of the green-brier, after the birds abandoned the nest, made it in many cases inaccessible.

During my almost daily visits to these bush-retreats of the white-footed mice, I determined one fact about the density of these growths of smilax, as late as October: that the small hawks, and even smaller shrike, found sparrows and mice quite out of reach when they took refuge therein. In one case, a sharp-shinned hawk, a little more rash than usual, struck at a snow-bird, as the latter dived into some opening in the briers, and, instead of capturing it, the hawk was himself hopelessly entangled.

Four of the forty-two nests occupied by the mice were placed in clusters of blackberry-canes, a growth which proved to be by no means easy to penetrate, but probably would offer no serious obstacle to a determined foe, but certainly could not have been suddenly assaulted—a condition which rendered the occupants comparatively safe.

The remaining seven nests were in a mixed tangle of Virginia creeper and grape-vine. These seven nests were all at a greater elevation than any I found in smilax or other thorn-bearing growths; one nest being thirteen feet from the ground.

These bush-retreats of the mice were all distinctly globular, or globoid, with the entrance usually near the rim of the original structure, and looking downward. These original structures were not merely covered at the top, but distinctly arched over; and the exterior often had a "pulled-to-pieces" appearance that suggested a chance accumulation of twigs and dead leaves, rather than designed.

A careful examination convinced me that twenty-nine were nests of cat-birds, cardinal-grosbeaks, or song-thrushes, and two were nests of the robin; all of which had been built by these birds in May or June of the same year—five or six months previously. The nests of the robin are apparently less popular, on account of the partial or complete mud-lining.

In this series of twenty-nine nests I determined that the foundations and lower portion of the sides, for about one half their extent, were unaltered to any significant extent; and many appeared as if a smaller bird's nest had been bodily removed, inverted, and so used as a roof to the lower structure. Whether the rougher exterior, to which I have alluded, was due to expoure since early summer, to unskillful work on the part of the mice, or a design of these mammals to render the nests less conspicuous, could, of course, not be determined.

Eight of the series were to me quite unlike birds' nests in their construction. The interlacing of the twigs was not like the ordinary work of birds; and the internal capacity of each one of this series was much smaller than that of an ordinary cat-bird's nest; while the exterior measurements were the same or nearly so—thus showing great difference in the thickness of the walls of the structure.

Three of these eight nests I picked to pieces, and the lining proved to be a mass of downy feathers—how they got them is a mystery—and an abundance of the "silk" of the milkweed; this being a material not used by any of the birds I have named, and indeed not ready for use until after bird-nesting is over.

Still, I am not yet prepared to make an ex-cathedra statement that these mice do build bird-nest-like structures in smilax and other dense growths, without having at least the base of a bird's nest as a starting-point; yet, why they should not, does not readily appear, when we remember that they build beautifully designed nests in hollow logs, tufts of grass, and under flat stones. Such nests are their ordinary summer homes.

It is certain that the materials for these summer nests, which, as a rule, are on or very near the ground, are often carried from quite distant points; so, why should they not carry them up a few feet into tangled growths, offering almost as sure a footing as the ground itself?

To recapitulate: judging from the number of nests examined—of course, another such series might give different results—the winter retreats or bush-nests of the white-footed mice are usually modified birds' nests; but in some cases the modification appears to be extended to practically a new construction.

Once within their nests, the white-footed mice are not readily disturbed during the day; and, unless the smilax or other growth is greatly agitated, they will not even take the trouble to look about them. By gently cutting my way toward the nest with a pair of shears, snipping here and there a branch or two, and drawing others gently aside, I have never failed to successfully surprise the timid occupants in their snug retreats. It is fairly safe, therefore, to conclude that I procured a pretty accurate knowledge of the number of occupants of each nest, the relative proportion of the one to the other sex, and of old and young. Thirty-six nests contained each a female mouse, and of these twenty-two were associated with young able to walk, while the others were burdened with the care of helpless offspring but a few days old. In not a single instance did I find a male mouse in these nests, while in the six other nests each was found to contain a single adult male mouse and no other occupant.

This unsocial condition of affairs seems to me the more strange, as in several nests placed upon the ground—many such nests are occupied the year round—both parents were found. They were not accompanied by any offspring, however; and it would seem as though a separation took place on the birth of a litter. Such facts tempt one to theorize, but I desist.

It was a pretty sight to see the mice when forced to quit their airy quarters in a thicket of smilax. Be the vine ever so slender, they took no uncertain steps, but tripped lightly down from point to point, and never arriving at a confusing corner. One female mouse turned just twenty times before she reached the ground. Once there, although she had proceeded very cautiously before, she suddenly disappeared. This, indeed, is always the case; but just where they go when they reach terra firma remains to be shown.

The prevalent impression is that every mouse has a subterranean retreat directly beneath the nest in the bushes, and passes from one to the other as fancy dictates. Their actions seem to bear out the truth of this, but I have never been able to discover such underground retreats in positions that conclusively showed they were frequently visited by the bush-dwelling mice above them. On the other hand, I have found the evicted mice to take shelter under dead leaves, pieces of bark, or limbs of trees. If disturbed from such lurking-places, they very seldom attempt to re-enter the elevated bush-nests, but scamper off over the weedy, leaf-strewed meadow.

Besides reconstructing nests of birds as dwellings for themselves, they convert others into magazines stored with carefully selected acorns, chinkapins, hazel-nuts, and corn; and so there really seems to be no necessity for bush-dwelling mice ever to return to the ground when once they have taken up their quarters in a smilax thicket—that is, return before the winter is over.

In this my experience is quite the opposite to that of others who have found underground retreats beneath the bush-nests, and have seen the mice, when forced to leave the latter, take refuge in them. It is possible, certainly, that these were burrows of the meadow-mouse (Arvicola riparia), and it would be hard to prove, in a meadow everywhere tunneled by mice and shrews, that the presence of burrowings, whether deep or shallow, beneath nests in bushes, was not merely a coincidence; and, again, I am quite sure the same tunnels are often used in common by widely different species of small mammals.

The stores of food for winter use are of much interest as connected with the subject of hibernation; but I can at present merely outline what I have seen, and what conclusions I have reached from such observations. It can be truthfully said that, while the white-footed mouse is not a hibernating animal, nevertheless it frequently hibernates. In other words, its prolonged sleeping, sometimes extending over several weeks, depends not upon the temperature, for I have seen them scampering over the snow when the mercury was nearly at zero, but upon their access to the food they have laid up for winter use. Cut this off and they will not starve, but pass into that curious torpid state which, with many mammals, continues for the entire season. I have experimented so frequently with them in regard to this, that I feel warranted in saying that one wonderful capability of the creature is, to be able to avoid starvation and its attendant horrors by optional hibernation.

Why, it will probably be asked, do so many of these mice quit their cozy quarters in or on the ground, and which have served them every purpose, and take all this trouble to build a new home in the bushes for the winter? It has been suggested that the nest was worn out, and better fitted for entomological research than for hesperomoid habitation. I had myself thought of this, but have never detected such abundant evidences of this disastrous condition as would warrant the removal; and certainly the fur of these creatures would carry, in all cases, a sufficient number of acari to bring about, in a brief space, a repetition of the plague.

The supposed excessive dampness during autumn and winter of many situations where the summer nests of the mice abound has also been urged as a probable reason for the marked exodus that, as we have seen, occurs on the approach of cooler and wetter weather; but the exposure to sudden summer showers would, in this respect, be more objectionable than the steadier rains and gradual melting of snow during winter; when, as a matter of fact, they are less apt to suffer from water encroaching upon their nests than at other times—the frozen condition of the rough surface tending to carry off the water and prevent its soaking into the ground. I have never found a nest that could not have been better guarded from the damps of winter than from those terrific cloud-bursts that recall the vivid description in Genesis of the Noachian deluge. During such rainfalls, for which the month of August is noted, very many white-footed mice are drowned.

From such scanty observations as I have been able to make, I am led to believe that the habit of such removals from the ground to the bushes has been brought about by the greater exposure to the attacks of enemies, when nesting upon the ground; these enemies being weasels, minks, and crows.

The two mammals I have named are certainly more given to prowling about the haunts of the mice in winter than in summer; and the crows, particularly when the ground is frozen, have often been seen tugging away at the unyielding stones or wood that sheltered such mice as had concluded that their present quarters were so favorably conditioned as to prove effectual against the assaults of whatever enemy might chance to come. The fact that the poor creatures sometimes suffered from an error of judgment led me to conclude that the representatives of the weasel family, that I have mentioned, and the omnivorous and omnipresent crow, are ever eager to capture white-footed mice whenever an opportunity occurs.

Probably years of further observation will prove necessary to clear up this important point of the cause that led to the habit of utilizing abandoned birds' nests; but I have no doubt that the question of comparative safety of the two situations, the ground or a thicket of smilax, had much to do with it.