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 



