Birds and rodents destroy different nests: a study of Blackcap Sylvia atricapilla using the removal of nest concealment

Nest predation is a major factor limiting the reproductive output of small passerines (Ricklefs 1969). Thus, selecting safe nest-sites is critically important for these birds. Nest concealment can inhibit transmission of visual, chemical or auditory cues to predators (Martin 1993). However, although there are numerous studies demonstrating the positive effects of concealment on nest survival (reviewed in Martin 1992), other studies were unable to find such an effect (e.g. Willson & Gende 2000). There are at least two reasons for the mixed results. First, we typically do not know the identity of nest predators. Different predators can select for different aspects of nest concealment, with the net result that there is no single best ‘value’ of concealment, and nests differing in their concealment may be depredated at similar rates (Filliater et al. 1994). Moreover, densities of different predators can fluctuate both among years and among sites, with net selection varying temporally and geographically. Secondly, the majority of studies have simply used a correlational approach. However, as the effects of concealment may be confounded by other nest-site features or with parental quality, an experimental approach is needed to separate independent effects of concealment. I am aware of only two experimental studies of the effects of concealment on nest survival (Howlett & Stutchbury 1996, Stokes & Boersma 1998). In this study I investigated the effects of concealment, height and season on nest survival in the Blackcap Sylvia atricapilla. I used natural Blackcap nests baited with artificial plasticine clutches to identify predators. I manipulated concealment by removing part of the foliage obscuring nests and tested whether different types of nest predators (birds vs. rodents) selected for different degrees of nest concealment.