50 



SCIENCE. 



[N. S. Vol. XXIV. No. 602. 



Stebbins ^ and Carpenter.^ It was found that 

 the migrating birds flew much lower than has 

 hitherto been supposed, most of them being 

 less than 1,500 feet from the ground at the 

 time of observation. 



The writers of the present article have re- 

 cently used the same instruments for the 

 purpose of measuring the speeds of migrating 

 birds. The theory of the method is simple 

 enough. Observer F. with a three-inch tele- 

 scope, was stationed 200 feet south of S., who 

 used a four-inch instrument. On seeing a 

 bird, F. would call ' time ' when it left the 

 disk, and S. estimated the interval in seconds 

 until the bird had passed off the moon's edge 

 from his point of view. By drawing a figure 

 it may easily be seen that in the observed in- 

 terval, the bird had crossed a space not less 

 than the distance between the stations. The 

 lines joining the two observers with the moon 

 were parallel and 200 feet apart, measured 

 horizontally from south to north. From the 

 time required to pass 200 feet, the speed of the 

 bird in miles per hour was derived. 



We were successful with this method on 

 only one night during the last migrating sea- 

 son. May 18, 1906. F. at the south station 

 called ' time ' for ten different birds, of which 

 three were seen by S. At the signal ' time,' 

 S. would count seconds to himself: 'and, one, 

 and, two, and,' etc., the first ' and ' being a 

 half second after ' time,' another half second 

 interval to ' one ' and so on. Both of us have 

 had some years' experience in observing 

 transits of stars by the ' eye and ear method,' 

 and we believe these results more accurate 

 than could be obtained with a stop watch. 



Two birds crossed the space of 200 feet in 

 one second, and the third in a second and a 

 quarter. The data and results may be summed 

 up in the following table where a correc- 

 tion of 0.2 second has been added to the ob- 

 served interval to allow for the velocity of 

 sound. 



^ Stebbins, Joel, ' A Method of Determining the 

 Heights of Migrating Birds,' Popular Astronomy, 

 14, 65, February, 1906. 



^ Carpenter, F. W., ' An Astronomical Deter- 

 mination of the Heights of Birds during Nocturnal 

 Migration,' The Auk, 23, 2, April, 1906. 



MAY 11, 1906. WIND S.W., 14 MILES PER HOUR. 



Bird 1 2 3 



Time of observation 11.30 11.38 11.50 



Time to travel 200 ft. (in sec.) 1.0 1.25 1.0 



Corrected time 1.2 1.45 1.2 



Speed in miles per hour 114 94 114 



It should be remembered that these are 

 minimum speeds, for if the birds were not 

 traveling due north they passed over more 

 than 200 feet between observations. From 

 the directions of their projected paths we 

 would conclude that they were actually flying 

 northeast, but we devoted nearly all of our 

 attention to the estimate of times. We con- 

 sider these results to be correct within ten or 

 fifteen per cent., and, therefore, place the ob- 

 served minimum speeds between 80 and 130 

 miles per hour. To obtain the actual motion 

 through the air, these quantities should per- 

 haps be reduced by fourteen miles per hour, 

 the velocity of the wind as shown by an 

 anemometer. The birds were flying nearly 

 with the \yind. 



To one who has not tried this method, the 

 question at once presents itself: How was it 

 known that both observers saw the same bird? 

 When one has kept his eye fi:sed at the end 

 of a telescope for five or ten minutes seeing 

 nothing but the moon, and a bird appears 

 within a second after his companion calls 

 ' time,' there is no doubt in his mind that one 

 and the same bird was seen. Moreover, it is 

 possible to show that the path seen by the 

 second observer was in prolongation of that 

 recorded by the first. 



It is unfortunate that we did not secure 

 more observations. The observers inter- 

 changed places and watched for another half 

 hour, but no more birds were seen from both 

 stations. In all F. saw about thirty-five birds 

 and S. fifteen during the period 11^ 20™ to 

 12^^ 20™ on the above night. We tried on the 

 two succeeding nights, but there were passing 

 clouds, and apparently fewer birds were flying. 



So far as we know this is the flrst time that 

 two telescopes have been used to determine the 

 speed of birds at night. Professor F. W. 

 Very,^ working at the Ladd Observatory, 



^ Very, F. W., ' Observations of the Passage of 

 Miaratincc Birds across the Lunar Disk on the 



