Short Communication a New Technique to Monitor the Flight Paths of Birds

The study of the mechanisms which govern bird orientation and navigation is severely hindered by the difficulty of reconstructing the route taken by the bird. Several methods have been used to study bird flights: direct observation using binoculars; following the bird by aeroplane or helicopter (Griffin, 1943; Hitchcock, 1952; Fiaschi, Baldaccini, Ioale & Papi, 1981); using a transmitter carried by the bird (Michener & Walcott, 1966; Schmidt-Koenig & Walcott, 1978); and location of the bird using radar techniques (Eastwood, 1967; Papi & Pardi, 1978). All these techniques require that the bird and the monitoring equipment always be within optical or electromagnetic 'visibility'. In this paper we propose a technique whereby the direction in which the bird is heading during flight is detected and stored, at pre-fixed time intervals, by a device attached to its back. When the bird is retrieved, the stored data, when processed (assuming adequate hypotheses) allow a reasonable reconstruction of the route followed. The proposed device measures the angle 6 between the horizontal components of the earth's magnetic field and of an axis of the device, coincident with the main axis of the bird's body. The device consists of a traditional compass equipped with a new transducer to convert the angular values into electrical resistance values which entrain the frequency of an oscillator. Obviously, the knowledge of function d(t), where t is the sampling time, is not sufficient to characterize unequivocally the route followed. It does, however, constitute a useful point from which the flight path can be traced by an iterative and interactive process. Solid-state compasses were not taken into consideration because an acceptable sensitivity is associated with large weight and size.