The effect of limited visibility on vigilance behaviour and speed of predator detection: implications for the conservation of granivorous passerines M. J Whittingham, S. J Butler, J. L Quinn and W. Cresswell Whittingham, M. J., Butler, S. J., Quinn, J. L. and Cresswell, W. 2004. The effect of limited visibility on vigilance behaviour and speed of predator detection: implications for the conservation of granivorous passerines.  / Oikos 106: 377  /385. 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 invigilance 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 havebeen 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. M. J. Whittingham, S. J. Butler and J. L. Quinn, Edward Grey Institute of Field Ornithology, Zoology Dept, South Parks Road, Oxford, UK, OX1 3PS (mark.whittingham@zoo.ox.ac.uk). SJB also at: Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire, UK, SG19 2DL.  / W. Cresswell, School of Biology, Bute Building, St Andrews, UK, KY16 9TS. The level of predation risk faced by animals can increase for many reasons (decreasing group size, predator presence, distance from refuge, Elgar 1989, Quenette 1990, Lima and Dill 1990; plumage colouration Go ¨ tmark 1993; visual obstruction Metcalfe 1984, Cress- well 1994). Animals can compensate for this increased risk by being more vigilant. Vigilance is generally defined as time spent with the head raised during periods of foraging. Although ‘head-up’ periods may serve additional functions, such as scanning for conspecifics (Coolen et al. 2001) and handling food items (Illius and Fitzgibbon 1994), all models of antipredator vigilance Accepted 24 November 2003 Copyright # OIKOS 2004 ISSN 0030-1299 OIKOS 106: 377  /385, 2004 OIKOS 106:2 (2004) 377