Radio Transmitters Tail-Mounted on Hawks

Abstract

Birds were first monitored by means of radio telemetry more than 15 years ago, an early example being the use of back-mounted transmitting equipment to measure the respiratory rate of a flying Mallard Anas platyrhynchos (Lord et al. 1962). Although transmitters have been mounted on the backs of diurnal birds of prey (Southern 1964, Platt 1973, Fuller et al. 1974) and owls (Nicholls and Warner 1972, Forbes and Warner 1974), the body harnesses used for mounting can entangle the feet of hawks (Nicholls and Warner 1968). Moreover, back-mounted transmitters reduced activity and food-consumption of captive Red Grouse Lagopus lagopus scoticus (Boag 1972). Another problem with this transmitter mounting technique can be the premature breakage of weak links in the harness which have been incorporated to ensure its eventual detachment. To avoid the use of body harnesses, transmitters may be attached to the proximal end of tail feathers (Bray and Corner 1972). Equipment weighing less than two percent of a bird of prey's weight has been mounted on a single rectrice without causing the feather to fall out (Dunstan 1973, Fuller and Tester 1973). There is then no speed to make special provision for removal of a transmitter, because it will be shed at the next moult. This paper describes a technique used for tail-mounting transmitters in studies of Goshawk Accipiter gentilis behaviour and winter predation in Britain (Kenward 1976) and Sweden, and of Sparrowhawk Accipiter nisus movements during the breeding season. Radio-tagged hawks are compared with those marked only by leg rings, in terms of weight changes before retrapping, tendency to disperse, and hunting performance. There is no evidence that the tail-mounted transmitters affected the hawks' behaviour.