Radioactive Tagging of Animals Under Natural Conditions

The biochemist's use of radioactive isotopes for tagging molecules is well known and widely appreciated; but these newly available materials can also be used to advantage in ecological studies which involve tracing the position and movements of individual animals. Harmless radioactive tags permit many types of ecological and behavior studies to be carried out under natural conditions without the presence of a human observer. I wish to describe one successful application of this method to homing experiments with wild birds, and suggest the scope of its potential usefulness in other types of investigation. In most studies of the homing of birds it has been necessary for an observer to watch the nest of each bird throughout the daylight hours between its release and its return. When planning such experiments in the Arctic it was clear that the continuous daylight would increase the difficulties of visual checking; and an automatic recording device was therefore developed to register the time of return of an individual bird, as distinguished from its mate or others of the same species. Radioactive tagging offered the great advantage of specificity; no natural event and scarcely any type of apparatus failure could give a false indication of the marked bird's return. The radioactive material was attached to the conventional aluminum bird band, and a recording radiation meter was operated near the nest to measure the slight rise in radiation intensity at the time of a tagged bird's return. The actual instrument consisted of a conventional radiation detector (Nuclear Instrument and Chemical Corporation Model 2610) with an extra stage of amplification so that 0.2 milliroentgen per hour produced a fullscale deflection of an Esterline-Angus Model AW recording milliammeter. The clockwork of the meter and the dry batteries for the amplifier would operate eight days without attention. The instruments were housed in a metal box and the Geiger-Mueller tube and cable were protected by rubber tubing, so that the whole assembly operated well even when half submerged in water. The recording meter drew a baseline the position of which was determined by the intensity of natural background radiation; if the line rose to a clearly higher level the return of the tagged bird to its nest was registered in unequivocal fashion. The apparatus was used primarily with the semipalmated plover Charadrius semipalmatus; but it would be equally applicable to any other situation in which it was desirable to record an animal's arrivals and departure within a small area. For tagging animals in this way isotopes emitting gamma rays are preferable to alpha or beta ray emitters, because of the greater range at which gamma rays can be detected after penetrating through any solid material that may come between the tag and the detecting instrument. Furthermore, alpha and beta radiations increase the radiation dose inflicted on the tagged animal without materially facilitating the instrumental detection of the tag from any considerable distance. Hence in this paper only gamma radiation will be considered as a means of detecting the radioactive tag. The method has two inherent limitations: only one animal at a time can be 1 This work was supported by a research contract between the Office of Naval Research and Cornell University, and was based at the Arctic Research Laboratory, Point Barrow, Alaska. This aid is gratefully acknowledged, together with the cooperation and encouragement of Dr. Laurence Irving.