Alien predators pose a particular threat to ground-nesting birds in Northern Europe

Alien species are recognised as presenting an increasingly global threat to native species, but we often lack evidence of their interactions and effects. For example, boreal nesting duck species have recently declined in abundance and in Finland more than half of the species are now classified as endangered. Alien predators have been suspected as playing a part in these declines and new studies suggest that this suspicion is justified. A joint Helsinki and Aarhus University study established more than 400 artificial nests containing farmed mallard (Anas platyrhynchos) eggs in normal duck nesting sites in Finland and Denmark, monitoring nest fates with wildlife cameras.

The invasive alien raccoon dog (Nyctereutes procyonoides) proved to be a more frequent and opportunistic predator than any of the native predator species. In Finland, raccoon dogs were more common nest predators than red foxes (Vulpes vulpes), although in Denmark the situation was reversed, possibly reflecting the species local relative abundance: the raccoon dog already ranks second/third among the most numerous species killed annual by Finnish hunters. The studies showed that raccoon dog was the most common mammalian predator of artificial nests in all duck nesting habitats, predating nests on lake shores as well as in forests, agricultural landscapes and in urban areas, whereas the presence of individual native mammal predators was restricted by habitat type. The other alien species, American mink (Neovison vison) was less frequent at artificial nests as the primary predator, but often visited nests already depredated by others. Corvids, which regularly broke eggs at artificial nests, were often the primary predators. Nests with broken eggs significantly attracted mammalian predators, hence inter-guild facilitation can contribute to an enhanced impact from diverse predatory communities equating to more than the sum of that of their constituent species. Both alien species not only take eggs, but also readily depredate sitting hens.

While it is clear that artificial nest experiments cannot directly demonstrate an effect of alien species on duck populations, the decline of nesting populations of many waterbird species in Finland coincides with increasing abundance of raccoon dogs and American minks and it seems unlikely that other ground-nesting birds are not suffering similar attentions from these predators . Controlling such predator species and preventing their dispersion, especially alien species like raccoon dog, is heavily resource demanding but has been achieved in Sweden. The success of such coordinated and concentrated control suggests that such conservation actions could have the potential to improve boreal duck breeding success, while bringing potentially far wider ecological benefits.

Read more:

Holopainen S, Väänänen V-M, Fox AD (2020) Artificial nest experiment reveals inter-guild facilitation in duck nest predation. Glob Ecol Conserv e01305.

Holopainen S, Väänänen V-M, Fox AD (2020) Landscape and habitat affect frequency of artificial duck nest predation by native species, but not by an alien predator. Basic Appl Ecol 48: 52–60.

Krüger , H , Väänänen , V-M , Holopainen , S & Nummi , P 2018 , ‘ The new faces of nest predation in agricultural landscapes : a wildlife camera survey with artificial nests ‘ , European Journal of Wildlife Research , vol. 64 , no. 6 , 76 .

Holopainen S, Väänänen V-M, Vehkaoja, M, Fox AD. Do alien predators pose a particular risk to duck nests in Northern Europe? Results from an artificial nest experiment. Online early, Biological Invasions.

Pöysä H, Linkola P (2021) Extending temporal baseline increases understanding of biodiversity change in European boreal waterbird communities. Biol Conserv 257: 109139.

Modern ways for studying migratory birds

People have long attempted to understand the behaviour of migratory birds. The life of seasonal birds appeared to be mystical. People therefore invented all kinds of reasons why birds vanished from time to time. As little as two hundred years ago, people in Nordic countries believed that, swallows overwintered in the bottoms of lakes, although theories about migration began popping up at the time. Barnacle geese were named after crustaceans called barnacles, as people in English-speaking Europe thought the geese were born from these barnacles. The geese disappeared for the summer, but barnacles were seen instead. Barnacle geese breed in the high Arctic, so gooselings were obviously never seen in Central Europe.

Bird ringing or banding began in Denmark in 1899, when Hans Christian Cornelius Mortensen marked his first starlings. Ringing became an important tool for bird research, aiming to study migration in particular.

Researcher Ib Krag Petersen continues a long tradition of Danish bird ringers. A red-breasted merganser (Mergus serrator) ringed in Iceland was controlled from her nest.

Ring discoveries help understand migration routes and phenology in addition to bird habitats. However, birds usually need to be captured to read the rings. This commonly means that ring recoveries are from birds found dead or birds that have been shot. Thus, bird ringing and death locations are generally the only information gathered from ringing efforts. However, new devices provide researchers with the opportunity to follow birds year-round.


Light-level geolocators are small devices developed for following bird migration. Geolocators measure daylight, which makes it possible to define the times of sunrise and sundown. Calculating the longitude is possible by measuring the difference in the timing of noon and the Greenwich meantime, and calculating the latitude is possible by measuring day length. However, location data accuracy is rather poor. Geolocators do not send data, so birds must be recaptured to collect the devices and obtain the data. This means that the method is laborious and suitable only for birds that are highly likely to return to certain locations. Geolocators are therefore usually attached onto philopatric female birds captured from their nests. Size is a good feature of the geolocators. They can be tiny (even under 0.5 g), which makes them suitable even for small birds. A rule of thumb for transmitter weight is generally a maximum 5% of the bird’s weight. Geolocators may also be used to measure other variables such as temperature.

Geolocators are used for example in Iceland to study the wintering areas of diving ducks. Diving ducks winter at sea, so following their movements otherwise is difficult. Ducks are philopatric and so females captured from their nest sites will probably return to the same place year after year. Therefore, a female captured and marked at her nest can be found again presuming she survives the whole year. Collected geolocators reveal where females have spent the rest of the year.

Ib Krag Petersen’s study area in northern Iceland is searched every year to locate duck nests. All diving duck females are captured and checked for old geolocators and if none are found, new ones are attached to the ducks. This long-tailed duck (Clangula hyemalis) female got a geolocator on her leg for the first time.

GPS trackers

Trackers utilizing GPS technology produce the most accurate and timely information. GPS trackers potentially collect very accurate knowledge about bird location but can additionally measure many other variables such as the elevation and activity of the bird. GPS trackers can either send data or just collect it. Trackers may be set to send data even once a minute if the bird is within a telecommunication network. Sometimes these connections cause surprises, as a group of Russian eagle researchers found out. The studied eagle left its normal habitat and spent long times outside the telecommunication network. When it came back to an area with a network, all the data gathered during the blackout was sent at once to the researchers in the form of expensive text messages from Iran.

GPS trackers often use small solar panels, which enable data collection and sending for several years. Because GPS trackers are heavier than geolocators, their use is limited only to larger birds. Trackers may be attached as neck collars or as backpacks. Under-skin implants are also an option.

Antti Piironen uses GPS trackers to observe geese flyways and habitats along the flyways. Visit the link to follow the migration route of this graylag goose here.

Several bird species, such as birds of prey, black-throated loons and geese, are followed with GPS trackers in Finland. For example, GPS birds have been used to study wintering areas and changes occurring in these areas. Geese trackers have revealed details of a geese moulting migration to Novaya Zemlya. The movements of Finnish birds can be followed on the webpages of the Finnish Museum of Natural History and the Finnish Biodiversity Information Facility.

Read more:

Waterfowlers’ network 21.1.2020: Unraveling migration patterns of Finnish Greylag Geese

BBC 25.10.2019: Migrating Russian eagles run up huge data roaming charges

Finnish Museum of Natural History

Finnish Biodiversity Information Facility

Oceanwide Expeditions: Barnacle Goose


Why does Finland need a change in legislation concerning the raccoon dog?

Guest author: Heidi Krüger

The debate about the ongoing changes in Finnish legislation concerning the status of certain invasive species is reaching bizarre dimensions. People are mixing facts and fiction, becoming confused with terminology, and accusing the counter part of demonization.

Raccoon dogs (Nyctereutes procyonoides) are very cute, and their puppies are even cuter. Like their non-related look-a-likes raccoons, many people see a potential pet in them, and find it difficult to understand why anybody would like to kill these furry cuties. Maybe it was ok before, when people needed hats like Davy Crockett’s, but nowadays that fur is out of fashion, can’t we just let them be?

Wetland ecology group_University of Helsinki_raccoon dog2

Raccoon dogs (Nyctereutes procyonoides) are invasive in Finland.


The EU says: Harmful invasive alien species must be outed

As a part of the EU 2020 Biodiversity Strategy, regulation of invasive alien species entered into force in 2015. In 2017, the raccoon dog was included on the list of harmful invasive alien species, and now it is time for Finland to act on this matter. The Ministry of Agriculture and Forestry has proposed that the status of the raccoon dog, along with that of some other alien mammals, should be changed from game species to unprotected species.

This change in status would mean that hunting is allowed year-round, using light (e.g. night vision oculars) and that no hunting license is required. Use of poison or instantly killing traps would not be allowed for raccoon dog hunting, even though the former is allowed for the eradication of rats and the latter for example on American minks (Neovison vison) (another species listed as an invasive alien species by the EU).

Many nature conservation and animal rights organizations are now loudly arguing against this change in legislation. One commonly heard quote is: “hunters are demonizing raccoon dogs even though studies have shown them to be quite harmless as predators”.


Not so harmless after all

Recent studies have shown that the raccoon dog can be very effective at searching for food and can be responsible for the depredation of ground-nesting bird nests on the mainland and in the archipelago much more severely than previously thought. This was shown with wildlife cameras that captured raccoon dog behavior at the nests.

Wetland ecology group_University of Helsinki_raccoon dog3

Raccoon dogs predate the nests of ground-nesting birds.

Earlier studies relied on the fact that egg remains were seldom found in raccoon dog feces or on the clues left by predators at the depredated nest. Studies with cameras showed that raccoon dogs do not consume eggshells and nest predators cannot be identified by analyzing remains at the nests.

Some earlier studies have also tried to prove the harmlessness of raccoon dogs by predator removal experiments. Considering the capability of the raccoon dog in invading new areas, we have to wonder whether these studies were successful in removing the animals. It takes several years of extremely effective eradication before a decline in raccoon dog numbers is reached and an improvement is seen in the nesting success of birds, as shown in a study on a Finnish wetland.

Wetland ecology group_University of Helsinki_raccoon dog4

Wildlife cameras were used to show that raccoon dogs are effective at locating eggs in the landscape.

Raccoon dogs spread a multitude of diseases

The raccoon dog is an invasive alien species that has invaded and still is invading our country. The game bag has doubled in the last twenty years, and 2016 was the first year it exceeded 200 000 individuals. There is no natural balance that would limit its population, at least not yet. If we let the population grow so big that it will balance itself, what is the cost? How many frogs and lizards must be eaten? It is not only ground-nesting birds that suffer from predation, most amphibians are doing really badly at the moment, and they don’t need an extra predator either.

The raccoon dog has other harmful properties besides predation. Its presence enabled a serious rabies outbreak in the 1980s in Finland, and we are still spreading vaccinated bates along the eastern border of the country to keep the disease out. And there are other diseases to consider. The raccoon dog is a host for scabies, trichinella, and canine distemper virus. Finland has so far not had any cases of Echinococcus multilocularis, which has already reached the Baltic countries. The raccoon dog will be a very effective vector for its spread and this will have tremendous side effects in a country where practically everyone collects berries from the forests and eats them unwashed.

So, the need to control this species really exists. It is not just the hunters who “hate” the species or who try to blame it for the extinction of birds instead of admitting that habitat loss is a real issue that has to be dealt with.


Let’s unite in a campaign for biodiversity

The current situation where raccoon dogs are protected during the summertime helps the population gain its previous strength year after year, as we now have seen in Finland. Yes, some raccoon dog pups may lose both parents and succumb to hunger, but as studies have shown, 90% of the pups will not survive their first winter anyways.

The worries of non-hunters engaging in raccoon dog hunting and employing inappropriate methods to kill these animals are most probably exaggerated. Anyone who has ever met a raccoon dog either in the wild or trapped in a cage, knows they are not as defenseless as often pictured. Euthanizing them requires skills and an effective gun. In Finland, owning a gun usually requires a hunting license. Besides, unprotected species are also covered by animal protection laws, so illegal or unethical methods are not allowed.

The change in legislation will help arrange an effective campaign over larger areas. It will make it easier for hunters to hunt this species. It will lessen the burden of the raccoon dog hunt that is currently placed on local hunters. It will motivate hunters when society finally accepts the removal of this species that does not belong in our nature.

And maybe, with the introduction of an intensive removal program, we can also motivate landowners to restore more habitats. This could help many species hovering on the brink of extinction, or even more, bring back those we lost many years ago. This would mean truly working to increase biodiversity, which is the large issue behind this ongoing debate.


References and reading:

Dahl, F & Åhlén, PA (2018) Nest predation by raccoon dog Nyctereutes procyonoides in the archipelago of northern Sweden Biol Invasions 21: 743.

Kauhala, K, Kowalczyk, R (2011) Invasion of the raccoon dog Nyctereutes procyonoides in Europe: history of colonization, features behind its success, and threats to native fauna. Current Zoology, 2011

Kauhala, K (2004) Removal of medium-sized predators and the breeding success of ducks in Finland. Folia Zoologica 53 (4), 367-378

Krüger, H, Väänänen, V-M, Holopainen, S, Nummi, P (2018) The new faces of nest predation in agricultural landscapes—a wildlife camera survey with artificial nests. Eur J Wildl Res (2018) 64: 76.

Sutor, A, Kauhala, K, Ansorge, H (2010) Diet of the raccoon dog Nyctereutes procyonoides—a canid with an opportunistic foraging strategy. Acta Theriologica

Väänänen V-M, Nummi P, Rautiainen A, Asanti T, Huolman I, Mikkola-Roos M, Nurmi J, Orava R, Rusanen P (2007) The effect of raccoon dog Nyctereutes procyonoides removal on waterbird breeding success. Suomen Riista 53:49–63 in Finnish with English summary

Government proposal:

Raccoon Dog IUCN Red List:

Proposal for management measures and pathways of invasive alien species (2019): Ehdotus haitallisten vieraslajien hallintasuunnitelmaksi ja leviämisväyliä koskevaksi toimintasuunnitelmaksi

Game bag statistics:

Cellulose digestion in insects

There are approximately one million insect species in the world, but only 78 are known to digest cellulose. Termites are probably the most known and most efficient cellulose digesters. Although cellulose digestion is rare, it occurs in 20 insect families, indicating that it has evolved several times during evolution.

But why does cellulose digestion even matter? Cellulose digestion is important in forests, because the decaying of wood and other plant materials would be impossible without it. Without cellulose-digesting organisms, dead wood and plants would fill up our forests. Cellulases (which are cellulose-digesting enzymes) have been found in many herbivorous animals, including mammals, birds, reptiles and several invertebrates. Cellulose digestion by white rot fungi is the most precisely understood and three different enzymes operate during the process. However, characterizing the cellulases in insects is challenging. Most cellulase enzymes found from insect guts are not produced by the insects themselves, but by microbial symbionts. Researchers have found certain active enzymes that break down crystalline cellulose. In this way, insects can have active cellulose digestion of their own.

Fomes fomentarius

Fomes fomentarius, also known as the tinder fungus, belongs to white-rot fungi. © Mia Vehkaoja

Four types of cellulose digestion occur in insects: exploitation of symbiotic protozoans in the hindgut, utilization of bacteria in the hindgut, usage of fungal cellulases received from food and production of own cellulases. Lower termites and wood roaches use protozoans to digest cellulose, whereas higher termites digest cellulose with the help of bacteria. Beetle larvae and siricid woodwasps get digestive enzymes from their food. But the production of insect cellulases is rare and contested.

The controversy of insect cellulases is based on the low value of cellulose as an energy resource. Insects get most of their carbon from sucrose and other oligosaccharides. Cellulose is difficult to digest, so it does not provide any extra advantages to insects. Nitrogen and water are additionally crucial resources affecting insect growth and reproduction. So what benefits could cellulose digestion provide insects?

Insects with gut symbionts benefit from protozoans and bacteria, as they do not have to produce the cellulases on their own. They receive some of their carbon intake from cellulose, which will increase their energy supplies. The same advantages are apparent in the fungal cellulases that insects receive from their food.

As there is little evidence of insect cellulases, more studies need to be conducted. Gene technology is one alternative for finding watertight evidence. If we could isolate and clone the cellulose genes, we may be able to find the processes behind cellulose digestion. This would put an end to the speculations. Findings of insect cellulases could also bring new information of insect and deadwood relationships.

Knowing whether insects that secrete their own cellulases prefer certain wood types would be interesting. For example, could they select certain tree species instead of other species? Or could individual insect species even be differentiated to individual tree species, as fungi flies are to certain fungi species? Maybe we will have answers to these puzzling questions in the future.


Beetles and slime moulds – together forever

Slime moulds are organisms that use spores to reproduce. Slime moulds were previously classified as fungi, but are nowadays considered a polyphyletic group with several supergroups. None of the supergroups belong to the fungi. The interactions between slime moulds and beetles are very old: group Agathidium is the largest beetle group that mainly feeds on slime mould fruiting bodies. The round fungus beetle (Agathidium pulchellum) is one of the rarest and most poorly known European species in the genus. The species is very small, under four millimetres. The elytra, i.e. hardened forewings, are heart-shaped and black. Slime mould Trichia decipiens is the main host of the round fungus beetle and the beetle is classified as endangered (EN) in Finland.

Trichia decipiens is a common slime mould species worldwide. It depends on the dead wood of conifers and broadleaf trees. The round fungus beetle was recently discovered to prefer deadwood-dominated environments, especially old-growth forests dominated by spruce. The round fungus beetle was previously believed to associate with aspen, but more information is needed on the species preferences. The conservation and management of an endangered species is successful only once enough knowledge is available on their biology and habitat requirements.

A study conducted in Kuhmo, eastern Finland attempted to find all the slime mould species that the round fungus beetle associates with. In addition, the study wanted to discover which tree species T. decipiens grows on, and whether it is possible for the round fungus beetle to live in managed forests. Forests in Kuhmo are dominated by Scots pine (Pinus sylvestris) and intensive forestry is practiced in the region. Rapid loss of old-growth forests is occurring as a result of these practices. The study sites were restricted to spruce-dominated forests, with approximately half of the sites located in nature reserves and the other half in managed forests. In addition to slime mould and beetle surveys, the researchers determined the number of decaying logs and their decay stages, along with the density of each tree species.

In total, 26 slime mould species were found in the nature reserve sites and T. decipiens was the most common slime mould species found in early summer. Spruce was the main tree species for T. decipiens (65%), but it was also found on birches, aspen and pines and once from alder. T. decipiens preferred large-sized logs in middle decay stages (DC2-DC4). The round fungus beetle was only found on T. decipiens growing on spruce, birch and aspen. The beetle occurred mainly in nature reserves, although once in a managed forest site, but this site was not managed ordinarily. A preference for sites with log densities over 30 m3 per hectare was a limiting factor for the round fungus beetle occurrence. As predicted, the quantity of dead wood was higher in nature reserves than in managed forests.

Although three other common slime mould species (Hemitrichia clavata, Trichia persimilis and Trichia varia) were found in the Kuhmo region, the round fungus beetle was only found from T. decipiens. The average dead wood volume in Fennoscandian managed forests is two to ten cubic metres per hectare, and T. decipiens requires at least 10 cubic metres per hectare, so dead wood volume restricts the occurrence of T. decipiens in managed forests. T. decipiens is a spore-dispersed species, so the quantity of available substrate limits its occurrence. Managed forests do not have enough available substrates to disperse to.

T. decipiens is most likely the main host species of the round fungus beetle and it occurs in old-growth forests with high conservation value. Quite little is currently known about the habitat requirements of the round fungus beetle. The species occurs in only a few localities in Finland, but it may be more common than previously believed. T. decipiens, the host species of the round fungus beetle, is very hard to find, because its sporangia are very small and it lives under tree trunks and bark.

The loss of black-headed gull colonies disrupts other waterfowl species

Black-headed gull (Chroicocephalus ridibundus) colonies are loud and observant. Numerous birds are observing the landscape, making it difficult for predators to creep up unnoticed. Gulls will make alarm calls immediately when they observe something suspicious, and attack the invader – regardless of whether it is a crow or a human. The colony offers gull chicks a safe place to grow.

Wetland ecology group_University of Helsinki_naurulokki

Black-headed gulls breed in colonies. Gulls will make alarm calls immediately when they observe predators close by.

Certain animal species may depend on each other in nature. For example, certain bird species set up their nesting places near the safety of aggressive gull colonies. Dependence can lead to trouble, if the protector species disappears for some reason.

Many duck species utilise the protection of gull colonies. Nests within colonies survive better than nests outside. For example, the pochard (Aythya ferina) and tufted duck (A. fuligula) suffer from higher nest predation rates outside gull colonies. The tufted duck in particular prefers nesting within gull colonies. On the other hand, dabbling ducks do not usually nest near the colonies, but later during the breeding season they will take their broods close to the colonies.

Wetland ecology group_University of Helsinki_punasotka

The pochard has declined dramatically in Finland. Nest predation is one probable reason behind this trend.

A recent study measured the connections between different waterfowl species and gull colonies. The researchers evaluated the trends of several waterfowl species at 15 Finnish lakes and observed that the population patterns of the other species tend to follow the trends of black-headed gulls. Lakes that lost their black-headed gulls also began losing other species. The researchers showed that  the species declining the most were nesting in the same environments as the gulls. This was reflected in their conservation status: species connected to gull colonies were in a worse conservational status than generalist species or species nesting somewhere else.

Wetland ecology group_University of Helsinki_tukkasotka

Aythya species benefit from nesting near gull colonies.

Thus, the loss of gull colonies appears to lead to declines in other species. Colonies have traditionally protected nests and broods. Breeding season is a sensitive time for ducks, which may suffer from high predation rates during this time. The loss of gull colonies leads to duck species being exposed to increased predation.

Why then are the gulls disappearing? The exact reasons are unknown, but the species is suspected to also suffer from elevated predation pressure. Even black-headed gulls cannot protect their nests against every predator species. For example, the alien species raccoon dog (Nyctereutes procyonoides) and American mink (Neovison vison) have invaded Finland, and both are suspected to destroy gull colonies. Black-headed gull colonies have disappeared from many traditional nesting places, affecting the breeding success of other waterfowl species. Gull colonies are therefore far more important for  breeding ducks than earlier believed.

Read more:

Pöysä, H., Lammi, E., Pöysä, S. & Väänänen, V.-M. 2019 Collapse of a protector species drives secondary endangerment in waterbird communities. Biological Conservation

Veli-Matti Väänänen 2000: Predation Risk Associated with Nesting in Gull Colonies by Two Aythya Species: Observations and an Experimental Test. Journal of Avian Biology

Väänänen, V.-M., Pöysä, H. & Runko, P. 2016: Nest and brood stage association between ducks and small colonial gulls in boreal wetlands. – Ornis Fennica