Nest predation by raccoon dog Nyctereutes procyonoides in the archipelago of northern Sweden.
The raccoon dog is a medium sized canid native to East-Asia. It was introduced to the western Soviet Union during the first half of the twentieth century, and has since then spread to, and established in, many European countries where it now is considered invasive. Raccoon dogs are suspected to have negative impacts on biodiversity, for example through nest predation, but empirical evidence is scarce. In this study we used GPS monitoring combined with camera traps on both artificial and natural nests to find out: (1) if raccoon dogs find and scavenge eggs from artificial nests, (2) if the scavenging from raccoon dogs is additive or compensatory to the scavenging from native species, and, (3) if raccoon dogs actively scare brooding birds off their nests and prey on their eggs. We found that raccoon dogs effectively located artificial nests and scavenged their eggs. There was a significantly higher scavenging frequency on experiment islands with both raccoon dogs and native scavengers, than on control islands with only native scavengers. There was no difference in native scavenging frequency on islands with versus without a raccoon dog, suggesting an additive effect from the raccoon dog on top of the native scavenging. GPS-tracked raccoon dogs moved intensively in the archipelago during the bird breeding season, swimming long distances to reach new islands if needed. Raccoon dogs that arrived on islands with natural nests actively scared brooding hens, up to the size of graylag goose, off their nests and preyed on their eggs. Raccoon dogs preyed on all the eggs they found, but discarded the egg shells. Not consuming the egg shells consequently leads to few visible traces of eggs in their stomachs or faeces, which in turn may explain why egg predation by raccoon dogs has been largely overlooked in previous studies. We discuss the potential impact of raccoon dogs on biodiversity, in the light of our new findings, and conclude that the raccoon dog may have a much larger effect on the breeding success of ground nesting sea birds than what has so far been the predominating view in the scientific literature.