New Method for Measuring the Flight Altitude of Birds

Abstract

WE have measured the flight altitudes of homing pigeons and swifts using a new type of altimeter carried by the birds. The altimeter is based on the fact that the range in air of α-particles from a radioactive substance is inversely proportional to the density of the air and that air density decreases in a regular way with altitude. The altimeter used with the pigeons consists of a lucite tube with outer dimensions of 50 × 8 × 5 mm³ (Fig. 1). The α-particle source (²¹⁰Po) is protected by a thin plastic film and inserted through slits in one end of the tube. The detectors are sheets of cellulose acetate plastic placed in the two 45° slits in the other end of the tube, making it possible to measure the maximum range and range distribution of the α-particles. The altimeter weighs about 1.0 g and is open at both ends so that the air may pass through freely without significant heating. Experiments performed by the Aeronautical Research Institute of Sweden have shown that pressures inside and outside the tube are nearly equal, representing an error in altitude of only 10 m (too high). There is no systematic error in the air density determinations. The precision in a determination of the maximum range depends on the source strength and the duration of the flight. The error amounts to about 0.2–0.4 mm, which corresponds to an error of about 100–200 m in flight altitude in the homing pigeon experiments.