Long-term monitoring data reveal effects of age, population density, and environmental aspects on hatching success of Common Cranes (Grus grus)

Facing climate and land use change, a species' ability to successfully adapt to changing environments is crucial for its survival. Extensive drainage and intensification of agriculture and forestry set wetlands and associated species at risk of population declines. The population of Common Cranes (Grus grus) has experienced considerable fluctuations over the last century. Despite increasing population numbers, hatching success seemed to have decreased over the last years. The aim of this study was to identify factors influencing hatching success and nest survival of Common Cranes based on analyses of long-term individual-based monitoring data from northeastern Germany and evaluate the species ability to adapt to changing environments. Hatching success decreased over the course of the study period from 0.75 to 0.55. Surprisingly, nest survival and hatching success did not vary across different nesting habitats, whereas factors such as female age, timing of nest initiation and breeding pair density were found to have significant effects on hatching success. Older females showed higher hatching success, even though the proportion of unhatched eggs was highest in females aged 20 years or older. Early nest initiation had a positive effect on hatching success. Water levels are more favorable early in the nesting season, whereas increasing evaporation with time causes water levels to decrease, granting easier access for predators. Independently of female age, hatching success decreased with increasing numbers of breeding pairs within a 2-km radius around a nesting site. High population densities intensify competition for resources and promote intraspecific interactions, affecting reproductive outcome negatively. This study gives first insights into mechanisms behind population regulation in Common Cranes, highlighting the importance of population dynamics and individual features. We suggest to further investigate density dependent effects including landscape and habitat features as well as reproductive success in terms of chick survival, since successfully raising juveniles is crucial for a species survival.