Nest-boxes for raptors as a biological control system of vole pests: High local success with moderate negative consequences for non-target species

Biological control of rodents in agricultural areas, increasing avian predator abundance by nest-box provisioning, has been proposed during the last decades in several regions around the world as an alternative to the widespread use of anticoagulant rodenticides (AR) to protect crops. However these experiences have often lacked a “Before-After-Control-Impact” (BACI) experimental design providing evidence strong enough about their efficacy reducing vole abundance or crop damages. Furthermore, little is known about possible indirect effects on non-target species.Here we test the effectiveness of providing nest-boxes to common kestrels (Falco tinnunculus) and barn owls (Tyto alba) in reducing the abundance of two different vole species using indirect index methods to estimate rodent abundance in late spring (May). We monitored the abundance of both vole species in a treatment (with nest boxes) and control (without nest boxes) areas during three years in alfalfa fields, using a BACI design. We studied also the efficiency of this control method in three different crops (alfalfa, cereal and fruit-trees) during the last year of the experiment. We compare the results obtained in this study area in NE Spain with those obtained simultaneously in other three experimental areas in NW Spain. Finally we explored potential negative effects of this method on the abundance of several passerine birds present in our research area.Our results showed a clear-cut reduction in the abundance of the two vole species, the common vole (Microtus arvalis) and the mediterranean-pine vole (Microtus duodecimcostatus) in the experimental area after applying the treatment (nest box installation) in alfalfa crops. We also found a significant decrease in the presence of both species of voles on fruit-tree plantations and alfalfa crops but we did not find significant effects in the cereal fields, where voles were in general very scarce. These results suggest higher efficacy of biological control in this study area than in some areas of NW Spain, what could be explained by several non-exclusive hypotheses presented in the discussion.For other species, we found a negative effect of nest-boxes on the abundance of a limited number of common bird species, but also positive effects over other species using poles with nest-boxes as perches.In conclusion, under the conditions of our study area, nest-box provisioning seemed to be an effective technique to reduce vole abundance, but also had some undesired side-effects on non-target species. However, these side-effects affected mainly to common bird species and can thus be considered less relevant than those caused by SGARs, often affecting to endangered rodent predators and that could thus have negative effects over any potential future natural control of vole abundance, potentially increasing future needs of SGAR use in a vicious circle.