The Big Four reign in eutrophic lake reedbeds: predation on artificial waterbird nests

Nest predation is one of the main factors shaping waterbird abundances, but the impact of different predator species varies considerably between habitats and sites. To identify nest predators and evaluate their spatio-temporal impact on waterbird broods, we carried out a study with artificial nests and camera traps placed in the reedbeds of two eutrophic lakes in northeastern Poland. We also analyzed the importance of two factors for nest survival—water depth at nest site and egg concealment. Artificial nest survival was very low, with only about 8% of the nests surviving more than 10 days. Among the four most frequently recorded nest predators that comprised 95% of all predators filmed, the raccoon dog Nyctereutes procyonoides dominated (51.7% of records), followed by the marsh harrier Circus aeruginosus (24.5%), hooded crow Corvus cornix (12.2%), and American mink Neogale vison (11.6%). The predation rate of the raccoon dog significantly increased over time, being highest in July, and was opposite to that of the marsh harrier, whose nest predation was highest in April and declined over the next months, while the predation rates of the hooded crow and mink were stable. The concealing of eggs did not significantly decrease the probability of depredating nests. Water depth also did not significantly affect nest survival, and only the raccoon dog’s predation rate was negatively related to water depth. Birds depredated nests during the day and mammals both during the day and night; however, activity patterns of four main predators highly overlapped.