Scaring waterfowl as a management tool: how much more do geese forage after disturbance?

Abstract

With increasing numbers of many herbivorous waterfowl species, often foraging on farmland, the conflict with agriculture has intensified. One popular management tool is to scare birds off the land, often in association with shooting. However, the energy costs of flying are considerably higher than those of resting. Therefore, when birds fly off after a disturbance, they use extra energy that subsequently needs to be compensated. We used the white-fronted goose Anser albifrons, the most common (grass-eating) goose wintering in western Europe, as a model species. We measured flight durations by high-frequency accelerometer recordings over 2 × 24 h in nine focal geese that were only incidentally disturbed. We also made direct observations on these days to determine whether the flight durations were reliably recorded. Using both a simple and a more realistic model of the energy balance, we calculated the extra grass consumption resulting from additional intentional disturbances. On average, the geese flew daily 2 × 323 s (from and to their roosting sites at 3200 m) and furthermore took to the air 5·3 times during a day (and 1·9 times a night). Multiplied with the average flight durations of 195 s, this gives a total flying time of almost 0·6 h day−1 and a total foraging time of 7·4 h day−1. The extra foraging time needed to compensate for additional intentional disturbances strongly depends on the frequency of such disturbances and the following flight duration. If, for example, flights when intentionally disturbed are twice as long (2 × 195 s), the extra foraging time will be 3·7% day−1 (2·3–3·2% day−1 in the more realistic model) for each intentional disturbance, and the geese will no longer be able to cover their energy requirements when intentionally disturbed six times per day. Synthesis and applications. Recent experiments suggest that geese have to be scared frequently in order to reduce goose visitation to particular fields. With an intentional disturbance rate, for example, of five times a day, the birds' compensation for the increased energy expenditure will lead to a higher overall consumption of grass of 11·5–16% day−1. Accommodation schemes have to take such increases in total grass consumption into account when deciding on the refuge areas to be set aside.