The effects of habitat on the vigilance of shorebirds: Is visibility important?

Abstract

Previous studies of vigilance have concentrated on situations where the prey species has an unimpaired view of its surroundings. Here the effects of reduced visibility caused by objects adjacent to the prey are studied in two species of shorebird. A reduction in visibility causes an increase in the level of vigilance, indicating an increase in vulnerability despite the greater degree of camouflaging. This increase is due to individuals being less able to see both approaching predators and their neighbours. Turnstones, Arenaria interpres, and purple sandpipers, Calidris maritima, show very similar increases in the level of vigilance with decreasing visibility, but achieve these increases by different means: turnstones lengthen the duration of each vigilant scan, while purple sandpipers scan more often. Increasing scanning rate produces a shortened interval between scans, which reduces the risk of being caught unawares by an approaching predator. However, it may also reduce feeding efficiency, and it is suggested that this might be more serious in turnstones due to greater handling times for food items, so causing them to adopt a different strategy to increase vigilance.