Nest predation pressure on Chinese Bulbuls decreases along the urbanization gradient in Hangzhou, China

Nest predation is an essential factor affecting bird population density, reproductive ecology and life cycle. However, there are still contradictory results about how nest predation pressure changes with urbanization. One of the reasons for the controversy is that previous studies often use artificial nests to investigate nest predation, but the result of artificial nests is often biased and contrary to that of natural nests. Therefore, it is important to perform nest predation experiments simultaneously with both natural and artificial nests. In this study, we examined the change of nest predation pressure on the Chinese Bulbul (Pycnonotus sinensis) along the urbanization gradient in Hangzhou, China. We detected 148 natural nests of Chinese Bulbuls and continuously monitored them in two continuous breeding seasons from 2012 to 2013. After the breeding season, we placed artificial eggs in natural nests of Chinese Bulbuls to investigate the changes in nest predation pressure and used infrared cameras to record the predators. We then calculated the urbanization synthetic index (USI) as the measure of the degree of urbanization and examined its relationship with nest predation pressure. We found that no matter whether natural nests or artificial eggs were used, the nest predation pressure always decreased significantly with the degree of urbanization. The average height of shrubs within 10 m of the nest differed significantly between the predated and unpredated nests, which also affected the risk of nest predation. Our study highlights the importance of using natural and artificial nests simultaneously to conduct nest predation experiments, which can reduce the bias or errors caused by only using artificial nests. We also recommend the use of infrared cameras in future nest predation studies, which has obvious advantages in monitoring and identifying potential predators.