The effect of limited visibility on vigilance behaviour and speed of predator detection: implications for the conservation of granivorous passerines

Foraging animals frequently change their pattern of vigilance when they move from a patch with a clear view to one with a relatively obstructed view. This has been widely interpreted as compensation for a reduced ability to detect approaching predators in obstructed habitats. We test the extent to which changes in vigilance may compensate for the effect of reduced visibility on an animal's ability to detect predators. We measured the vigilance, foraging and speed of predator-response behaviours of lone chaffinches Fringilla coelebs that fed on seeds (800 per m(2)) scattered on artificial stubble habitats (with either a clear view of surroundings or an obstructed view). On both treatments, individuals with more rapid head-up rates responded more quickly to a flying model sparrowhawk Accipiter nisus (P=0.04); as did individuals with shorter food-search times (P=0.02). However, neither head-up rate nor food-search period varied with obstruction. Based on research previously published using this system, we suggest that this is because individuals are constrained in their ability to increase head-up rate because doing so is largely determined by their individually determined foraging efficiency. Instead chaffinches increased the duration of their head-up periods by 13% in the visually obstructed treatment (long stubble) and peck rates were 13% lower. Despite this presumed attempt to compensate for reduced visibility, duration of head-up period had no effect on response time of the fast-moving predator in our experiment and birds were 24% slower to respond in the long stubble. Rather than being maladaptive, increasing head-up duration may have been related to enhanced detection of other predator types, for example stalkers. Our results have implications for the conservation of wild granivorous birds. They support the view that agroeconomic decisions that affect micro-habitat structure over a large scale could affect predation risk, habitat choice and the conservation status of granivorous birds.