Page:Auk Volume 37-1920.djvu/722

604 General Notes. [oS the track of a pilot-balloon released at this station to determine the upper-air currents on the afternoon of May 3, 1920. Since the birds appeared to be oriented in the same general direction and to be flying in compact group formation, I decided to take readings of the positions of the individuals which could be "spotted" in the telescope, with a view to determining the speed, direction and incidently the altitude of the birds.

Before giving the details of the observations I may state that no attempt has been made to come to a conclusion as to the kind of birds noted, but my belief is that they were either hawks or ducks, owing to the similarity of their mode of flight to that of the wild-goose but with more rapid beating of the wings than in the wild-goose, with whose flight I am familiar. At the distance at which the birds were observed neither color nor fine definition of type could be seen, although the spread of a single wing of the individuals seemed to approximate the size of the pilot-balloon which was last seen at about the same level as the birds were using.

Conditions were especially propitious for determining the altitude of the birds, for the clouds closely beneath which they were winging were of the cumulus type, with flat, equally elevated bases and domelike tops. Luckily the balloon rose into the base of one of these clouds and was lost to view at an altitude of 1700 meters.

Upon losing the balloon. I turned the theodolite against a background of cumulus cloud and awaited the arrival of an individual of the flock to come within the field. Some idea of the large numbers of the birds can be had from the fact that it was possible to pick up at random a space in the sky and promptly find one of the birds winging across it. The birds were more than a mile distant and efforts to see them with the naked eye were fruitless.

Both of the individuals sighted were kept in sight for 60 seconds; before a second minute-interval elapsed they had become immersed in cloud and lost to view. The first bird "spotted," whose altitude was assumed to be 1600 meters by reason of its passage under the cloud base, was picked up at azimuth 193.2° (0 equal to North, 90 equal to East), and at elevation 25.2°. Sixty seconds later it was found at azimuth 198.9°, and elevation 21.2°. The resulting track shows a ground speed of 13.2 meters per second toward azimuth 223° (SW).

The second bird was picked up at azimuth 182.3°, elevation 36.2°; sixty seconds later it was found at azimuth 189.2°, elevation 31.7°. Its resulting track shows a ground-speed of 9.3 meters per second toward azimuth 221° (SW), when an altitude of 1800 meters is assumed. This was arrived at from the fact that this bird flew into the edge of one cloud after passing indistinctly through the extreme lower side of another cumulus cloud.

As the first individual was encountering a north wind of 4.5 meters per second (as computed from the pilot-balloon run) his wing-speed was 7 meters per second. The second individual encountered a north-north-west wind of 4.0 meters per second at the 1800 meter level, hence its wing