INITIAL FLIGHT DIRECTIONS OF HOMING BIRDS

Abstract

The initial headings of homing common terns (Sterna hirundo) and Leach''s petrels (Oceanodroma leucorhoa) were observed for as long as possible after release in unfamiliar, inland areas. In the final experiments two observers 1/4 to 1/2 mile apart obtained cross bearings on each bird at 15 second intervals until it was lost to view. This procedure is recommended whenever the initial headings of birds are being studied, since it reveals the actual direction of flight with greater accuracy than recording only the bird''s bearing when it is lost to view. The petrels showed no consistent headings, but at all six release points the terns showed a consistent tendency to fly approximately southeast, provided the sun was visible. Under heavily overcast skies they showed random headings. In the four experiments when the most accurate bearings were obtained (with the sun visible) 43 terns showed headings which averaged 144[degree] with an average deviation of 16[degree]. The total range of these headings was 97[degree] (90[degree] - 187[degree]). When the home direction was 115[degree] two groups released on different days showed average headings of 140[degree] and 142[degree]; when home was at 44[degree] the average heading was also 142[degree]; and when the bearing of home was 211[degree] the average flight direction was 149[degree]. This southeasterly tendency is an example of what has been called type II or fixed direction orientation, and it is probably based on the sun. It may be advantageous for this population of terns to find themselves lost over land, since their summer range lias along a northeast-southwest coastline and flight to the southeast would always bring them back to the coast.