Cuckoo density as a predictor of functional and phylogenetic species richness in the predictive modelling approach: Extension of Tryjanowski and Morelli (2015) paradigm in the analytical context

The Common Cuckoo Cuculus canorus is a well known brood parasite bird species whose density is presumed to be correlated with taxonomic, functional and phylogenetic bird species richness. However, due to the complex interplay between environmental estimates and co-evolutionary processes, the power of this relationship is still debatable. Various attempts to create statistical models have not been conclusive, because multilevel interactions between Cuckoo density, environmental conditions, and host species richness measurements have not been addressed so far. Therefore, we extended the concept of the Cuckoo as an independent bird biodiversity surrogate, and instead of simply using its occurrence, we incorporated an index of its density as an additional predictor in the modelling procedure of avian biodiversity. We applied six different indices of biodiversity as bird species richness measures, i.e. the number of bird individuals, the number of host species richness, functional richness, functional evenness, functional divergence, and evolutionary distinctiveness. We generated two sets of Species Distribution Models (SDM) for each group of biodiversity measures. One set included Cuckoo density as an additional predictor, while the other did not. Having evaluated these models in the Random Forest approach, it turned out that Cuckoo density improved model performance in each case. Species density was positively associated with taxonomic diversity (total species richness and host species richness), functional richness and evolutionary distinctiveness, and at the same time it was negatively correlated with functional evenness and functional divergence of bird communities. Thus, we suggest that through co-evolution of relationships, the Cuckoo prefers habitats attractive to numerous bird species, especially phylogenetically unique host. From the applied ecology perspective, Cuckoo density can be considered as a quantitative measure of functional interaction processes in the SDM approach, leading to the identification of avian diversity surrogates.