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Raccoon dogs are a growing threat to waterbirds in Finland

More than half of the duck species in Finland are endangered to varying degrees, and the results of a new study suggest that raccoon dogs and other invasive species are to blame. Researchers at the University of Helsinki used wildlife cameras to investigate the causes of declining waterbird species. 

In collaboration with researchers at several universities, the Helsinki team conducted field experiments over the course of three years. The experts set up wildlife cameras which captured images of invasive species preying on artificial nests. 

Researchers in Finland and Denmark had previously built more than 400 artificial nests in natural duck nesting environments. About 290 of the nests, which contained farmed mallard eggs, were located throughout southern Finland. 

While raccoon dogs were found to be the most common nest predators in Finland, red foxes were the predominant predators in Denmark.

“The raccoon dog was the most common mammalian predator in all of the environments studied in Finland. As expected, raccoon dogs destroyed nests on shorelines, but surprisingly often also in forests close to wetlands,” said study co-author Sari Holopainen, a postdoctoral researcher at the Finnish Museum of Natural History Luomus.

The cameras revealed that many nests were visited by more than one predator, and that corvids were often the first to find a nest. If a corvid broke an egg in the nest, this attracted mammalian predators such as raccoon dogs and American minks.

“Our research shows that predators cohabiting the same area can benefit from one another. Eggs broken by corvids offer a clue to mammalian predators, which, in turn, are a threat to both eggs and female ducks. In other words, the effect of a diverse predator community can exceed the sum of its species,” said Holopainen.

The raccoon dog is a species that is native to Southeast Asia that was introduced to Europe as a fur animal beginning in the 1920s. The first raccoon dogs arrived in Finland in the 1930s. Today, they are the second or third most common prey animal.

The experts found that raccoon dogs have an advantage over native species because they are more versatile in terms of utilizing their environment. Native mammal predators are restricted by habitat type, but raccoon dogs were commonly observed on shorelines and in forests, as well as in agricultural and urban landscapes. 

“The decline of many waterbird nesting populations, especially in eutrophic lakes, matches temporally the increasing abundance of raccoon dogs and American minks. This came out as we studied long-term changes in aquatic birdlife between the periods 1951–1970 and 1996–2015 using datasets collected in the Häme region by Pentti Linkola, the late fisherman,” explained study co-author Hannu Pöysä, a principal scientist at the Natural Resources Institute Finland.

According to the researchers, the artificial nest experiments cannot directly prove that the raccoon dog has an effect on duck populations. 

“What the results do show, however, is that the raccoon dog is a more common nest predator than native mammalian species, and that it is abundant in a range of duck nesting environments,” concluded Holopainen.

The study is published in the journal Biological Invasions.

By Chrissy Sexton, Staff Writer

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