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

Though the occurrence of night time feeding has been demonstrated in numerous species of wildfowl, accurately quantifying this behaviour visually is inherently difficult and so alternative techniques are required. During the course of two distinct projects on the ecology of brent geese Branta bernicla in England (Studies A and B) and another in Germany (Study C) we used position-sensitive radio transmitters, attached to thin leather neck collars, to monitor feeding behaviour remotely. In this paper, we present a collective account of the method. Transmitter units emitted pulses at two different intervals depending on the orientation of a built-in tilt switch; generally 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 goose were required to produce predictive equations which described the proportion 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 could be used to interpret data that was received and stored at regular intervals (30 or 60 seconds) during night time by a data logging system. Each study also assessed whether the collars affected the behaviour of the geese, No significant behavioural differences were observed for free-ranging birds wearing collars compared to nearby birds without. This technique is a realistic option for ecological studies of wildfowl requiring construction of feeding time budgets through the night as well as the day.