Nest predation on an abundant generalist bird in tropical China

Nest predation is the main reason for breeding failure of small passerines. Theory states that predation in the incubation stage should be mostly due to predators finding poor-quality sites, whereas predation in the nestling stage may be driven by parental activity. Yet field evidence for this theory is mixed, and there are few studies from tropical Asia. Red-whiskered Bulbul (Pycnonotus jocosus; RWB) is an abundant generalist species in tropical Asia, often found in human-modified environments. Between 2012 and 2017 in the Xishuangbanna Tropical Botanical Garden, Yunnan, China, we monitored 775 RWB nests, including 287 nests with infrared-triggered cameras, 20 nests with digital video cameras, and 10 with temperature data loggers. The richness of nest predators was high, with at least 11 species, including mammalian, avian, reptilian, and invertebrate species; northern tree shrew (Tupaia belangeri) and Greater Coucal (Centropus sinensis) were the predator species most detected and identified. There was no significant difference in daily predation rate (DPR, as assessed by the Mayfield method) between incubation (DPR = 0.046, n = 682) and nestling (DPR = 0.049, n = 525) stages, even though parental visits in the nestling stage were almost 3 times the number in the incubation stage. Yet during the nestling period there was a strong positive relationship between the parental visitation on a particular day post-hatching (from day 1 to day 10; visitation measured with digital video cameras) and the nest predation rate of that day (measured on all nests). To determine the effect of nest sites on predation, we placed clay eggs in 127 previously used RWB nests. Sites that had been depredated at the incubation stage had marginally higher DPR (0.076, n = 43) when reused than sites that had been successful (DPR = 0.043, n = 50), although this difference was not significant when Bonferroni-corrected. This result thus provides weak evidence for an effect of nest site selection on nest predation. In total, this study emphasizes the strong forces of selection during the nesting period, particularly for birds in human-modified tropical habitats where the diversity of nest predators is high.