. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. FURTHER OBSERVATIONS OF HOMING TERNS 237 OBSERVATIONS OF INITIAL HEADINGS All of the terns were released under clear or partly cloudy skies when the posi- tion of the sun was evident, though the sun itself was sometimes temporarily hid- den behind a cumulus cloud. The day was warm, and there were numerous up- drafts on which the terns tended to climb and soar. We thus lost sight of some birds at heights of several hundred feet above the ground before they had indicated a definite heading away from the release point. Unknown
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. FURTHER OBSERVATIONS OF HOMING TERNS 237 OBSERVATIONS OF INITIAL HEADINGS All of the terns were released under clear or partly cloudy skies when the posi- tion of the sun was evident, though the sun itself was sometimes temporarily hid- den behind a cumulus cloud. The day was warm, and there were numerous up- drafts on which the terns tended to climb and soar. We thus lost sight of some birds at heights of several hundred feet above the ground before they had indicated a definite heading away from the release point. Unknown to us at the time the re- leases were begun, there was also a small pond miles to the southwest of the release point which attracted some of the FIGURE 2. Sample flight paths of eight common terns. Solid circles and triangles stand for crossbearings, open circles and triangles for single bearings. Directions of the home col- onies are shown by the arrows in the box (H) at the left. Positions of the observers are rep- resented by the corners of the large triangle. Contour interval is 100 feet. See the text for a discussion. The results are summarized in Figure 1, in which the points of the compass are "unrolled" on the horizontal axis; each bird is represented by a vertical rectangle. Only those birds are included that gave a distinct heading; the nine attracted to the pond and the seven lost while still circling less than 3000 feet from the release point have been omitted. Home direction is indicated by a capital "H" and a verti- cal line, the mean heading by an unmarked vertical line. The graph shows that the birds, from the Michigan colony at least, tended to head in a southeasterly direction. The average of the 19 Michigan terns was 139° (40^237°), while the average head- ing for the 6 Massachusetts terns was 93° (64—130°). In previous experiments. Please note that these images are extracted from scanned page images that may have been di
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
