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.
