INITIAL FLIGHT DIRECTIONS OF HOMING BIRDS

¹ The Biological Laboratories, Harvard University, Cambridge 38, Massachusetts

1. The initial flight directions of common terns and Leach's petrels were observed for as long as possible after release in unknown, inland areas. In some experiments two observers obtained cross bearings on each tern for as long as it could be seen through binoculars (maximum 12 minutes and two miles distance).

This procedure is strongly recommended whenever the initial headings of birds are of interest, since it reveals the actual direction of flight with greater accuracy than recording only the "vanishing point."

2. The petrels exhibited no consistent headings towards home or in any other direction, but one experiment resulted in returns from an inland release point similar in speed to those reported earlier from releases at sea or along the coast.

3. The terns showed a consistent tendency to head approximately southeast when the sun was visible. Average headings in the most satisfactory experiments were 140°, 142°, 142°, and 149°, while the total range of headings among 43 terns in these four experiments with the sun visible was 97°. The mean deviation from the average heading of 144° was only 16°. There was no significant difference between the headings when the direction of home was 44°, 115°, or 211°.

4. This southeasterly tendency disappeared almost entirely when the sun was hidden behind thick clouds, but it persisted under 90-95% cloud cover when the sun could be seen occasionally through breaks in the clouds.

5. The southeasterly tendency represents a special type of what has previously been called type II orientation, and it is probably based upon the sun. It may be a useful ability for terns that nest along the eastern coast of the United States, since whenever they find themselves inland, flight to the southeast will bring them quickly back to the coast.