More is better: predator dilution effect increases Chilean Elaenia (Elaenia chilensis) nest survival

Nest survival may be affected in several ways by predator-mediated interactions. According to the “predator dilution effect hypothesis”, the daily nest survival rate (DSR) can be density-dependent, because the probability of nest predation decreases as the number of active nests increases. Additionally, parental activity may increase with clutch size, nest age, and after low-temperature periods, while climatic components such as air humidity and moderate winds can increase odour dissipation from nests. Consequently, predators can use these cues to locate nests, reducing the DSR. Determining such predator-mediated interactions is important to understand how multiple causes can affect avian reproductive success. Thus, we evaluated the effects of predation and predator-mediated interactions of abundance of active nests, parental investment and climate on annual and seasonal variations in DSR of the Chilean Elaenia (Elaenia chilensis), a long-distance migratory passerine that breeds in the Andean-Patagonian Forest. We monitored nests over four breeding seasons and modelled DSR for 86 nests. The mean ± SE DSR was 0.960 ± 0.005, corresponding to an overall nest success of 29.5%. DSR increased with the abundance of active nests, but it did not vary among years or throughout the breeding season, nor was it affected by other variables. Hence, there was no influence of parental investment and climate on predation. Instead, the increase in abundance of active nests towards the middle of the breeding season reduces the probability of predation per nest, resulting in greater Chilean Elaenia nest survival, and providing empirical support for the “predator dilution effect hypothesis”.