< Page:Popular Science Monthly Volume 7.djvu
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THE BIOGRAPHY OF A BIRD.
they are indispensable in maintaining its position, at least when it is beginning its hole. We have further remarked that some of these martins' holes are nearly as circular as if they had been planned out with a pair of compasses, while others are more irregular in form; but this seems to depend more on the sand crumbling away than upon any deficiency in its original workmanship. The bird, in fact, always uses its own body to determine the proportions of the gallery—the part from the thigh to the head forming the radius of the circle. It does not trace this out as we should do, by fixing a point for the centre around which to draw the circumference: on the contrary, it perches on the circumference with its claws, and works with its bill from the centre outward; . . . the bird consequently assumes all positions while at work in the interior, hanging from the roof of the gallery with its back downward, as often as standing on the floor. We have more than once, indeed, seen a bank martin wheeling slowly round in this manner on the face of a sand-bank when it was just breaking ground to begin its gallery.
"This manner of working, however, from the circumference to the centre unavoidably leads to irregularities in the direction. . . . Accordingly, all the galleries are found to be more or less tortuous to their termination, which is at the depth of from two to three feet, where a bed of loose hay and a few of the smaller breast-feathers of geese, ducks, or fowls, is spread with little art for the reception of the four to six white eggs. It may not be unimportant to remark, also, that it always scrapes out with its feet the sand detached by the bill; but so carefully is this performed that it never scratches up the unmined sand, or disturbs the plane of the floor, which rather slopes upward, and of course the lodgment of rain is thereby prevented."

Sometimes the nest is carried to a far greater depth than two or three feet, as in a case observed by Mr. Fowler, in Beverly, Massachusetts, where, in order to get free of a stony soil where pebbles might be dislodged and crush the eggs, the tunnel was carried in nine feet, while neighboring birds in better soil only went a third as far. In one place the burrows will be close to the top of the bluff, in another near the bottom, according as fancy dictates, or the birds have reason to fear this or that enemy. English writers agree that occasionally their bank-swallows do not dig holes, but lay in the crannies of old walls, and in hollows of trees. This is never done, that I am aware of, in the United States; but in California a closely allied species, the rough-winged swallow, "sometimes resorts to natural clefts in the banks or adobe buildings, and occasionally to knot-holes." On the great Plains, however, our Cotyle burrows in the slight embankments thrown up for a railway-bed, in lieu of a better place,

"How long does it take the bird to dig his cavern under ordinary circumstances?" is a question which it would seem hard to answer, considering the cryptic character of his work. Mr. W. H. Dall says four days suffice to excavate the nest. Mr. Morris, a close observer of British birds, says, per contra, that a fortnight is required, and that the bird removes twenty ounces of sand a day. Male and female alternate in the labor of digging, and in the duties of incubation.

When the female is sitting, you may thrust your arm in and grasp

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