Biodiversity indicators for breeding passerines in a multiple-cuckoo system in China: A comparison of cuckoos and raptors

A total of 2337 species and subspecies of birds are distributed in China, including 820 species of Passeriformes, 19 species of Cuculiformes, and 99 species of raptors (Accipitriformes, Falconiformes, and Strigiformes) (Zheng 2017). To compare the indicative roles of cuckoos and raptors for passerine birds under a variety of climates, geographic regions, and ecosystems, we refer to Zhang's (1999) study to categorize the zoogeographical region of China into 19 subzones. These subregions are consistent with the regional differentiation of natural conditions in China (e.g., temperature, precipitation, monsoon, elevation, glaciers, and geologic changes), and each possesses a typical representative ecogeographical fauna (Zhang 1999). For example, the typical ecogeographical fauna of the Hainan Island subregion (VIIC) is tropical forest, forest and shrubland, grassland, and farmland fauna, while alpine forest-steppe, meadow-steppe, and fell-field fauna dominate the Qinghai-South Tibet- subregion (IVB). In contrast to artificially delineated administrative areas, such as provinces and cities, zoogeographic subregions can reflect the regional differentiation of animal distributions in both historical and ecological terms (Zhang 1999).We extracted the distribution data of birds from Zheng (2017) to the 19 subregions mentioned above. The causative mechanism based on predation relationships leads to strong ecological links between passerine birds and raptors, because raptors, such as sparrowhawks (Accipiter nisus), often feed on passerine birds (Burgas et al. 2014; Natsukawa 2020, 2021; Natsukawa et al. 2021; Sergio et al. 2005). Moreover, passerine birds are common hosts for cuckoos; hence, we used the diversity of passerine birds as an indicator, because of their strong ecological links to raptors and their function as hosts for cuckoos. we included all Accipitriformes, Falconiformes, and Strigiformes in China in the category of raptors. For all Passerines, Cuculiformes, and raptors, we selected only resident and summer migratory birds (e.g., birds that breed in each subregion) in the subregions, excluding winter migratory birds and straggler birds, to reflect representative birds adapted to the ecological and historical conditions of each subregion. For Cuculiformes, we excluded greater coucal (Centropus sinensis), lesser coucal (Centropus bengalensis), and green-billed malkoha (Phaenicophaeus tristis), which are non-parasitic cuckoos (Yang et al. 2012). To prevent changes in taxonomic status of passerines from affecting the subsequent calculation of phylogenetic diversity and functional diversity, we followed the bird list provided by Jetz et al. (2012), which follows the Birdlife V3 world list (June 2010, 9,895 extant species recognized), and treated the distribution data for Passerines as they were prior to the taxonomic change. Finally, we excluded the South China Sea islands subregion (ⅦE) from our subsequent analysis because the subregion is far away from the mainland, and bird surveys and studies are relatively absent. Therefore, we obtained the presence or absence of 765 species of Passerines, 17 species of parasitic cuckoos, and 99 species of raptors (including Accipitriformes, Falconiformes, and Strigiformes) in 18 subregions (ESM Table S1).The results showed that the number of cuckoo species had significant positive correlations with taxonomic diversity (TD), phylogenetic diversity (PD), and functional richness (FRic) of passerines but no significant correlation with evolutionary distinctness (ED) or evolutionary distinct and globally endangered (EDGE). In contrast, raptors showed a significant positive correlation only with EDGE. The greater number of factors associated with cuckoos suggests that they exhibit superior performance as biodiversity indicators compared to raptors. However, this does not undermine the significance of raptors as indicators. Selecting the indicator is context-dependent, with cuckoos being suitable for routine surveys, monitoring overall avian diversity, and raptors being suitable for assessing the status of endangered birds, conducting conservation measures, and measuring their effectiveness. We anticipate cuckoos to gain more public attention as a paradigm of biodiversity indicators and be put into practice.