Disentangling ground-nest predation rates through an artificial nests experiment in an area with western capercaillie (Tetrao urogallus) presence: martens are the key

The western capercaillie (Tetrao urogallus) is a species of conservation concern that has been experiencing a severe decline in many regions of Europe. The poor breeding success rather than mortality of full-grown birds has been suggested to be responsible for its decline, been the nest loss a potential driver of the low productivity (number of chicks per female and year) of capercaillie. Working in a capercaillie population in the Pyrenees, we aimed to provide further insights into nest predation by monitoring 82 artificial nests (58 of them with camera traps), and estimating the population density of predators using camera traps and applying random encounter model and camera trap distance sampling. Pine/stone martens -Martes foina, Martes martes- were the species with higher nest predation rates (23%) despite being the less abundant (0.49 ind/km²), followed by foxes -Vulpes vulpes- (11%) and a population density of 3.37 ind/km², and wild boar -Sus scrofa- (4%) and 6.95 ind·km^− 2; predators could not be identified at 27% of the events. After fitting statistical models on the nest predation data, we observed that the martens’ encounter rate (i.e. number of martens detected per camera and day) and the altitude had a positive significant effect on the artificial nest predation probability. While caution is recommended in artificial nest experiments, our results are valuable highlighting the importance of martens and red foxes when addressing nest predation to promote the breeding success and population recovery of western capercaillie populations.