Empirical evidence that diversionary feeding increases productivity in ground-nesting birds.

Abstract

The recovery of predator populations can negatively impact other species of conservation concern, leading to conservation conflicts. Evidence-based solutions are needed to resolve such conflicts without sacrificing hard-won gains for recovering species. Well-designed, large-scale field experiments provide the most rigorous evidence to justify new forms of intervention, but they are notoriously hard to implement. Further, monitoring scarce species without negative impacts is challenging, calling for indirect and non-invasive monitoring methods. Uncertainties remain about whether observational monitoring adequately reflects the true processes of interest. Having conducted a well-designed, large-scale, diversionary feeding field experiment that reduced artificial nest depredation, we evaluated whether this translated to capercaillie productivity in the same area. Using camera traps aimed at dust baths, we non-invasively monitored capercaillie hen productivity over 3 years and in 30 1km2 grid cells under a randomised control (un-fed) and treatment (fed) design. Diversionary feeding significantly increased the probability that a detected hen would have a brood. The impact of diversionary feeding did not change over the brooding season, indicating that hens without a brood had failed due to nest depredation rather than predation of chicks. The probability of detecting a hen with a brood was 0.85 (0.65-0.94) in fed locations, more than double that in unfed locations, which was 0.37 (CI 0.2-0.57). The average brood size was reduced over time, but the change did not differ between fed and unfed sites. This is in line with natural mortality occurring independently of diversionary feeding. Importantly, the increased chance of having a brood in the fed areas and the predicted brood size leads to a substantial increase in overall productivity - the expected number of chicks per hen - at the end of the sampling season. This was just 0.82 (0.35 - 1.29) chicks per hen in the unfed sites and more than double 1.90 (1.24 - 2.55) chicks in fed sites. This study provides compelling empirical evidence that diversionary feeding positively affects productivity, offering an effective non-lethal solution to the increasingly common conservation conflict where both predator and prey are afforded protection.