{Cuckoo and biodiversity: Testing the correlation between species occurrence and bird species richness in Europe}

The use of biodiversity surrogates is an increasingly popular tool, because it provides strong results while reducing the costs of conservation studies. Here, we hypothesize that cuckoo (Cuculus canorus) occurrence may correlate with high bird species richness based on the assumption that their presence should mirror the richness of their potential avian hosts and the overall bird community. Specifically, we assessed the association between species occurrence and taxonomic diversity patterns on a multi-spatial scale using datasets from seven European countries. Our results show that high bird species richness is a good proxy for cuckoo occurrence, and the best results were based on data from point counts. The species was almost absent at sites with low species richness, suggesting that the presence of cuckoo is an appropriate surrogate of bird biodiversity. The accuracy of the models ranged from 0.68–0.71 (for large spatial scale) to 0.86 (for local spatial scale) and provided valuable indications of bird taxonomic diversity distribution on all different types of environments monitored in each country. These associations are possibly related to co-evolutionary relationships with host species (correlated with overall species richness) and the cuckoo's preference for sites that are attractive to many other bird species, due to high habitat diversity or abundant food resources. Our findings highlight how conservation planners can use cuckoo occurrence as a surrogate to maximize efficiency when studying bird species richness patterns. These results also demonstrate the advantages of using the cuckoo rather than top predators as a potential surrogacy tool for citizen scientist programs.