The vigilance behaviour of two small tree gleaning birds differing in their spatial niches was studied in relation to variation in predation risk associated with distance to vegetation cover of the foraging patch. The Nuthatch (Sitta europaea; searcher on trunk and thick branches) and the Coal Tit (Parus ater; foliage gleaner) were used as focal species. The species studied adopted two different strategies while foraging at greater distances from cover: (1) the Nuthatch increased vigilance proportion (time spent vigilant per minute foraging), keeping the time spent at feeders constant, while (2) the Coal Tit decreased time spent at feeders, but showed no significant increase in vigilance proportion. The two components of vigilance (scanning rate and duration of individual scans) changed in different ways in the two species: the Nuthatch, which usually forages in open substrates, scanned more often, while the Coal Tit, which forages in substrates with denser vegetation cover, increased the length of scans (by reducing the proportion of short scans). These differences between vigilance patterns of both species are discussed in the light of differences related to the spatial configuration of the feeding niche (obstruction of visual space by vegetation) and the protection offered by the surroundings.
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