Long-term monitoring of common spadefoot toad activity in a European steppe using barn owl pellets

One third of the worldwide amphibian species are threatened, therefore, efficient monitoring efforts are needed. Amphibians which adopt a hidden lifestyle, such as the common spadefoot toad, are often missed with standard surveying efforts. Spadefoot toads can be identified in regurgitated pellets of the barn owl, which provides an effective way to estimate toad activity. In our study we analyzed frequency of spadefoot toad remains from 2004 to 2016 in a steppe landscape in eastern Austria.

We used an automated model selection procedure together with a GLM analysis using a zero inflated error Poisson distribution, to analyze the presence of Pelobates fuscus in barn owl pellets. All analyses were done in the statistical software R, and the scripts to reproduce our results are available within this publication. Our approach may provide a template for other researchers to use for their own pellet data.

Our analysis suggested that activity of the common spadefoot toad is mainly influenced by rainfalls, while time of the year and temperature had small but significant effects. Interestingly, our data confirmed the possibility of a second breeding period in summer, triggered by heavy rainfalls. There were no indications for a population decrease in the observed years and locations. Our study shows that barn owl pellets can be used effectivley to assess pelobatid activity in an area. This might constitute a useful monitoring tool for conservation management for amphibians.