Research methods

It is not easy to study small mustelids, which is clearly depicted by the limited number of scientific studies. A major reason for this difficulty is the complex ecology and elusive behaviour of the species, making it difficult to monitor or study them using standard field methods. The majority of classical studies is based on life-trapped individuals that could be marked or equipped with a transmitter. We advice to use non-invasive methods instead of life trapping as life trapping brings risks for the trapped animals and needs to be accompanied with the proper permits. The SMF uses the following methods for the different species:

Tabel l. Research methods SMF for each of the species. 

Method	Species
	weasel	stoat	polecat
‘Mostela’	xxx	xx-1	—
Tracking tunnel	xx-2	xx-2	—
Camera with/without bait station	x–	x–	x–
Tubecam	xxx	xx-1	x–

¹ The detection probability for stoat is as of today unsure, which is why we do not give this method a full score. However, notice that stoats have been recorded in Mostelas in several places within the Netherlands and abroad, suggesting that this method is successful in detecting the species.

² As tracks of weasel and stoat cannot always be distinguished, we do not give this method the full score. However, we highly recommend using this method when separation between the species is not necessary, or when using in combination with Mostelas or Tubecams.

‘Mostela’

Images of weasels and stoats from the field are rare. This is partly due to their movement speed and elusive behaviour, which makes it hard to capture them with a regular camera trap. 

The Mostela was specifically designed to capture weasel and stoat. It combines a regular camera trap with a tracking tube (ø 8 cm of 10 cm, PVC tube) in a wooden box (12 mm concreteplex). We expected that we would have a higher detection probability by combining the relatively successful tracking tubes with a camera trap, which would allow us to identify the species with certainty. We added a +2-diopter lens (obtained from a set of reading glasses) in front of the camera lens to decrease the focal distance, which is needed to get sharp images inside the Mostela.

The SMF currently sets the camera traps to record a video of 10 seconds with an interval of 30 seconds when triggered. We also regularly use bait, e.g., fish oil, in the Mostelas but want to stress that the use of bait should always be carefully considered as it might violate several assumptions of the statistical models often used to analyse camera trap data. Furthermore, bait can result in large amounts of footage of small rodents, and although bait can increase detection probability of small mustelids, it can also attract larger carnivores, that might scare away small mustelids from the Mostela. 

The Mostela can give certainty about the presence of certain species. Furthermore, the age and sex can often be classified from the images. Specifically for the common weasel (Mustela nivalis vulgaris) it is occasionally possible to identify individuals based on the spot pattern of the demarcation line. The original and current designs are based on, respectively, the Bushnell Trophy Cam and the Browning Strike Force HD Pro. If you want to use the Mostela with different camera models, it is important to test if the dimensions of the design need to be adjusted to the specifics of the camera model. 

The SMF currently uses Browning cameras in Mostelas and Tubecams due to their great quality for a relatively low price. You can find more information about these and other camera traps at https://www.wildlifemonitoringsolutions.nl/. The costs for material for making a Mostela are about €50.- (based on making 4 Mostelas simultaneously). 

Please check the following publications for more information about the Mostela:

• Croose, E. & S.P. Carter 2019. A pilot study of a novel method to monitor weasels (Mustela nivalis) and stoats (M. erminea) in Britain. Mammal Communications 5:7-12. The Mammal Society.
• Mos, J. & T.R. Hofmeester. 2020. The Mostela: an adjusted camera trapping device as a promising non-invasive tool to study and monitor small mustelids. Mammal Research. doi: 10.1007/s13364-020-00513-y.

Compilation Mostela

Tracking tube

Tracking tubes are often used to determine the presence of small mustelids. In New-Zealand (King & Edgar 1977) and Great Britain (Graham 2002), tracking tubes have successfully been used to determine the abundance of small mammals. The advantage of this method is that it is easy to use, non-invasive and relatively low cost (about € 3.- per tube). A tracking tube consists of a tube with a tracking plate, which has an inkpad embedded in the middle. On either side of the inkpad, the place is painted with a light colour on which the tracks can easily be seen. Note that it is important to use environment and animal friendly ink. 

A tracking tube can be made from a 50 cm long piece of PVC tube with a diameter (Ø) of 8 to 10 cm. A bigger tube can be used for stoat, but we found that stoats in captivity will go through a tube with a 8 cm diameter. In our study on the effectivity of the Mostela (Mos & Hofmeester, 2020) we found that the detection probability for weasel was larger with 10 cm diameter compared to 8 cm.

The tracking plate can be made from, e.g., thin birch wood (length: 49 cm, width: 5.5 cm for an 8 cm tube, and 8 cm for a 10cm tube), coated with an environmentally friendly lacquer. It is important to make the plate slightly shorter than the tube, to make sure it stays dry inside the tube. We use a piece of sponge cloth in the middle of the plate as inkpad (illustrations x and x). An environmental and animal friendly ink can be made from paraffin oil and ultra fine charcoal powder (illustration x). Do not use regular writing or printing ink, as this can be poisonous! Paraffin oil can often be bought at a local drugstore (around € 6.- per liter). Charcoal powder can be bought in bags of 100 g at Verfmolen De Kat (€ 5.75). Mix the paraffin oil with the charcoal powder (around 40-50 gr per liter) in a water tight bottle. Test if the mixture leaves a good track before you go into the field. If the print is too weak, you can add some charcoal until the print looks good. Always shake the bottle before use (!) and add some mixture to each tracking tube before deployment in the field, for example by using a paintbrush. To reduce problems due to snails and slugs eating the paper, we advise not to use paper to collect the tracks. Instead, use a light paint on which the tracks can be seen well. When checking the tubes, you can take a picture (with ruler for scale) to save the record and clean the tracking plate with a wet cloth.

Limitations of tracking tubes
Tracking tubes work really well as detection method. However, distinguishing between tracks of weasel and stoat is very difficult and sometimes impossible. This is especially problematic for male weasels and female stoats, that have overlapping dimensions of the tracks. A camera trap-based method, such as the Mostela or Tubecam, can help identifying the species. Thus, the SMF advises to use a combination of methods when monitoring small mustelids.

Camera trap

Originally designed for hunters and large game, camera traps are now used throughout the world to study a large array of wildlife. Depending on the research question and species, camera traps are deployed with or without bait or lure. While a scent station as lure can increase the detection probability of some species, it also limits the use of camera trapping data, as trapping rates are inflated due to the response of wildlife to the lure. However, especially for elusive or rare species, the increased detection probability is sometimes needed. Therefore, we use camera traps with and without bait or lure to detect polecats. 

Camera trap with scent station

There are several ways in which cameras can be used with a scent station. 

The ‘Jiggler’ was originally developed to capture the bib mark, or throat patch, of pine martens, which can be used for individual identification. However, it also appeared to be effective for other species such as the polecat. The ´jiggler´ exists of an iron rod with a tea egg at the end. Several types of bait can be used in the tea egg such as sardines or peanut butter. The iron rod can be pushed into the ground and positioned at various heights. We usually place it about 30 cm off the ground. The advantage of the tea egg is that it secures the bait so it functions as a lure. It is adviced to place the ´jiggler´at about 3 m from the camera trap. We have also used a variation in which the tea egg is placed on a small tripod which can be placed into a tree.

The polecat and pine marten (BUBO) of the Dutch Mammal Society uses a can of sardines on a stake in roughly the same setup as described above for the ´jiggler´. The so-called Struikrover^® can also be used to monitor small mustelids. This method is using a covered camera trap in a half-open tube with a can of sardines at around 50 cm from the camera. By using a lens in front of the camera, similar to the Mostela, it obtains sharp images at short distance.

Camera trap without scent station

It is not always necessary to use a scent station and as described above there are situations when it is not advised to use any bait or lure. It is also still unclear if polecats really respond to several types of bait or lure. Therefore, we think that placing the camera in a microsite where polecats are expected to occur is the best way to ensure a high detection probability. Examples are planks or small bridges over small waterways, small passage ways along streams, badger sets, underpasses under roads and favourable habitat.

Tubecam

We developed the tubecam based on the success of tracking tubes. We expect that these work so well based on the idea that weasel and stoat will curiously investigate any burrow that they find in their territory. The tubecam consists of a PVC tube (50 cm tube + cover for the camera) with a camera on one side. The advantage of the tubecam over the Mostela is that it can be placed in smaller spaces, which makes it easier to deploy in the field, especially in e.g., small ditches. The disadvantage is that animals are not captured from the side which makes it harder to identify individuals. We often apply the tubecam with fish oil (salmon) and similar camera settings as in the Mostela. Another advantage of the tubecam is that it can also photograph animals outside of the tracking tube, which makes it possible to detect polecat with this method.

 

 

Compilation Tubecam

Innovation and development

The SMF continues to work on the development of research methods, such as the Mostela and the tubecam. We work on the optimization of camera trap methods through trial and error. In this way, we aim to find effective methods to detect small mustelids, which can be used to study ecological questions or in monitoring.