Monitoring feeding behaviour of brent geese Branta bernicla using position-sensitive radio transmitters

Though the occurrence of night time feeding has been demonstrated in numero us species of wildfowl, accurately quantifying this behaviour visually is i nherently difficult and so alternative techniques are required. During the course of two distinct projects on the ecology of brent geese Branta bernic la in England (Studies A and B) and another in Germany (Study C) we used po sition-sensitive radio transmitters, attached to thin leather neck collars, to monitor feeding behaviour remotely. In this paper, we present a collect ive account of the method. Transmitter units emitted pulses at two differen t intervals depending on the orientation of a built-in tilt switch; general ly shorter intervals (ca 1.1 seconds) when a bird lowered its neck to feed and longer intervals (ca 1.4 sec ends) when in upright positions, though in some units this was reversed. Daytime observations of each radio-marked go ose were required to produce predictive equations which described the propo rtion of time feeding in terms of mean pulse interval (Study B), number of long intervals between pulses (Study C) or the proportion of time in which pulses were received at short (or in some cases long) intervals (Study A). Coefficients of determination ranged within 0.13-0.95. These equations coul d be used to interpret data that was received and stored at regular interva ls (30 or 60 seconds) during night time by a data logging system. Each stud y also assessed whether the collars affected the behaviour of the geese, No significant behavioural differences were observed for free-ranging birds w earing collars compared to nearby birds without. This technique is a realis tic option for ecological studies of wildfowl requiring construction of fee ding time budgets through the night as well as the day.