Page:Bird-lore Vol 01.djvu/343

 The Birds that Pass in the Night 115 out like a Duck, Loon, or Grebe. The slow passage was ev-idently caused by distance. No. 61 had flight like a Goldfinch. No. 72, wing-motions were plainly seen. No. 94 passed very slowly; this was evidently a large bird at long range, as it occupied three seconds in passing over the field and the wing-motions were plainly seen. We probably missed about one-tenth of the birds passing over the field while changing watches and changing eyes at the telescope, for it is impossible for the eye to focus steadily upon the bright surface of the moon for more than about five minutes, without ex- hausting the receptive power of the retina, so that a change of eyes is imperative. Being novices at the work, we probably missed more birds at first than after we had had the experience of a couple of watches; still it will be noticed that more birds were recorded for the first two hours than later in the night, which is proof that more birds were moving early in the evening. The number dropped off perceptibly at about 10.30. The Cartwright observatory, where these observations were made, is located back about three-fifths of a mile from the Detroit river, which at tiiis point is about a mile in width, and the telescope was pointed out over the river at an angle of about 30°. The fact that we were looking out directly over the river undoubtedly explains the general easterly flight of the birds noted. We would naturally expect a more direct northerly course of these spring migrants, but the birds were evidently following up the broad moon-lit course of the Detroit, which here runs from east-northeast to west-southwest, and we estimated that these birds were somewhat more than half a mile above the river. There are so many unknown quantities in making estimates from these observations, that it is impossible to arrive at any positive conclusions except as to the number of birds passing over the field of vision and their direction of flight. For instance, a bird that we noted as appearing to be in rapid flight might either be actually ia rapid flight at long range or might be in comparatively slow flight at short range. In either instance he would pass the field of vision quickly. Here the unknown quantity is distance, which, given, we could readily estimate actual speed. Then a bird flying at right angles to the direction of sight would appear to be moving faster than one moving obliquely toward or from the point of observation. A bird appearing large might be either a large bird at long range or a small bird at short range. Again, if a bird flying east should move upward or downward but a few feet while crossing the field of vision it would register the same result in the telescope as though he were moving northerly or southerly,- — that is, of course, when the moon is low in its orbit, as it is in the spring elliptic. There