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Wednesday, July 30, 2014

Small Mammal Trapping: Techniques for safe capture

If I asked most people to quickly name three mammals, I believe most of them would name larger mammals. Pets like dogs or cats, farm animals like horses or cows and zoo animals like elephants and giraffes would probably be common. Yet this is such a skewed sample of the mammals that currently live on this planet. Although our minds go to the more charismatic examples of mammals, the truth is that over 90% of mammal species are small. Merritt (2010) defines a small mammal as one weighing five kilograms or less. For my students and I, that translates to about 11 pounds. Woodchucks weigh about 11 pounds and are among the largest of the small mammals. This blog entry will focus on the smaller small mammals: Mice, small squirrels like chipmunks and shrews.

I am not currently engaged in any small mammal research (beyond simple inventorying of species) but I teach proper capture and handling techniques in my classes. Let's review the equipment first.
Sherman trap at a forest location
Naples, NY (7/14)
The photo at left shows a type of trap called a Sherman trap. Here is the link to the short entry on these traps in Wikipedia. Note that you can go to the company website from there if you want information on sizes, prices, etc. These traps have become the standard for live trapping as they are light, durable and fairly easy to clean. Importantly, they keep the trapped animal secure inside solid walls rather than a wire mesh cage. This provides the captured critter with more protection from weather and they may feel more secure as the closed trap is similar to a hole or burrow that most small mammals would seek for shelter. The flag serves several purposes. First, it makes the trap easier to locate. And by numbering the flags data collection is simplified. This is deployment #13 on the blue transect. Finally, if animals are not processed at the spot they are collected, a flag allows field workers to return the animal to the exact location it was captured.

Traps such as these require bait to increase the capture rate. I mean, a small mammal could stumble in and investigate, but bait is needed to really make this work. Much has been written regarding various bait effectiveness. We use the following recipe ( I believe credit goes to SUNY ESF???):
Sasha makes this for us but I believe this is a SUNY ESF recipe
A few things about this recipe. It is pretty simple and we have had good success with it. Other popular ingredients include raisins, molasses and nuts. It is difficult to break it into perfect squares, so we usually are faced with a plastic bag full of oddly shaped pieces. I tell the students to use bait about the size of a Starburst candy. There is really no need to use more. Traps are checked frequently and there is no chance the animal would need more food.
We also place cotton balls in the traps along with the bait. Cotton serves several purposes. It acts as insulation against the cold of the bare metal and it also gives the captured animal something to do. In my experience, captured rodents often shred the cotton balls and create a "nest".
Other useful equipment for setting traps includes plastic gloves for handling bait, GPS receiver for marking trap locations and a notebook. Please note that there is a different equipment list for checking traps.

To increase capture rate, we typically pre-bait traps for about a week. This entails setting the traps in their transects, adding cotton balls (I like to use two) and bait and leaving the back door open so any animal that enters the trap is free to leave. This can increase capture rate by getting the animals used to visiting the trap for a reward (bait). We try to re-bait every second or third day. It also gives us a chance to see the activity level in the area. If many traps are being cleaned out, we expect a bigger capture when the traps are finally set. One disadvantage of pre-baiting is that larger animals can also find your traps. We have had problems with raccoons, striped skunks and gray squirrels finding and raiding traps. Here are some examples:

Striped skunk raiding a Sherman trap
Canandaigua, NY (11/13)
Last November, we had several transects set on the main campus at FLCC. One was being raided. We set a camera trap and captured this striped skunk opening the trap and eating the bait. Sherman traps open like this to facilitate cleaning (there is a model that folds that does not open). In this case, pre-baiting allowed this skunk to find our traps and raid them numerous times during the week. To counter this, we used a small rope and two tent stakes to hold down each trap and the skunk was unable to get the bait.
Each May we offer a Conservation Field Camp course and small mammal trapping is part of that. Here, two technicians set and bait a Sherman trap specifically targeting flying squirrels.

This time it was a gray squirrel causing us problems:

A final example is more recent. We had traps set at our East Hill Campus in Naples for the past several weeks and one or more raccoons were hitting two of the four transects. Here, Sasha walks away from a trap after setting it. You can see that the back door is closed. The spring-loaded front door is facing away from the camera.

It only took about five and a half hours for this coon to visit. He found this set during the pre-baiting period. In this photo he is starting to reach into the trap.

It is difficult for him to reach the bait without triggering the trap. By now, I bet this guy has enough experience with these traps to know that if he pulls out his arm without the bait, he won't get a second chance as the door will close.

As mad as I get at them, this picture makes me smile. 

The rock wall transect was getting hit as well. Here is a pre-bait photo. Notice first that you are looking at an open back door. The back door is not spring loaded so animals are free to move in and out. To try to increase the camera trap captures at this photo, Sasha placed some natural bait (hickory nuts and mushrooms) on a rock.

Less than two hours later, an eastern chipmunk enters the trap and presumably takes some of the bait.

As hoped, there were lots photos at this location, including some mice in the genus Peromyscus. Check out this mouse in mid-leap with an open Sherman trap in the background. I thought "What a lucky shot! That'll never happen again." Well.....

... I was almost right. Several nights later, I got a photo of presumably the same mouse JUST ABOUT to make the same leap. So I did not get him mid-leap, but I did capture the same behavior a second time.

And then a third... Here I present them to you in the order they were taken. All were captured on different nights. But when I use these in class, I will be putting them in "order" to make it look like a sequence of three photos of one mouse making one jump :)
But back to marauding raccoons. Here is the last image of the trap with the back door open. Notice the edge of the opening. 

Since the last photo, Sasha has closed the back door, set the spring loaded front door, re-baited the trap and turned it around so that the opening is still in the same direction as it was during the pre-bait week. Can you see how the edge of the opening looks different than in the photo above? The front door opening has a bit of a lip that the door closes against.

Here is the very next photo captured. The door is closed and the mushrooms have been moved. Obviously, the camera missed some things.

Not far behind was this raccoon. When we checked the traps, this one was tossed open and empty. Was there a critter in the trap when the coon opened it? Probably, but we will never know for sure.

Once we capture a small mammal, the handling protocols we use are designed to assure a safe and quick experience. For example, shrews are particularly susceptible to trap-stress, so our protocol calls for processing them first. Students are instructed to keep noise to a minimum. We still talk, but we use our inside voices. Students work in pairs with one wielding the clipboard while the other processes the animal. And as mentioned previously, animals are returned to the exact location of their capture.
Let's start at the beginning. Students check all the traps in a given transect. If there is a small mammal captured in a trap, the trap label is marked with the flag number and color and taken to the processing location. Since the goal of the activity is to teach the students how to participate in all aspects of small mammal trapping and handling, we process the animals in a central location so we can all learn from all captures. You may wish to process animals right at the capture site depending on your research question.

Removing an animal from the trap is not difficult, but does take some practice.
Here, FLCC professor Clinton Krager places a large Ziploc bag over the back door of a Sherman trap. The bag opening is folded over so the animal cannot escape. We use plastic bags for two reasons. It is easier to see the animal in the plastic bag rather than a cloth bag and therefore makes the process go faster. The animals do not spend much time in the plastic bag so they do not run out of air or get too hot.

Sometimes an animal is reluctant to leave a trap even after shaking it a little. A nice trick is to aim the opening up. Often an animal will scoot right out.
Author removing a flying squirrel from a live trap
Photo credit: Alyssa Johnson
When the animal is in the bag, the bag and contents are weighed. Usually, the cotton and some bait are in the bag as well as the animal. Once the animal is removed from the bag, the bag and contents are weighed and subtracted from the total to find the true weight of the animal.

Weighing a mouse with a spring scale
Canandaigua, NY (4/14)

The next step is removing the animal from the bag. Here, Clinton holds the bag against his leg and uses one
hand outside of the bag to hold the animal while sliding his other hand onto the bag to grab the animal by the scruff of the neck. Again, the animal spends a short amount of time in the bag and the researcher can see the animal easily. I have also found that animals will latch on to cloth bags with teeth and claws making them harder to extract. This technique works very well with small small mammals. It does not work well with larger small mammals like gray squirrels. Oh, and flying squirrels have lots of loose skin and can be difficult to hold so that they cannot reach around and bite.

Sasha is holding an eastern chipmunk properly. This was her first chipmunk. They can be bitey, so she was using gloves. The data we collect is pretty basic. I mentioned weight. We record the species, gender, total and tail lengths and age (adult, immature or unknown).

If we are holding an animal for any length of time (to let students observe it), we place it in a jar. The jar
makes it easier to see and most animals calm right down when placed inside. Meadow voles will begin grooming almost immediately when put in a jar.
Finally, we have experimented with different methods of marking small mammals. Ear tags are available in various sizes and easily attached to most critters (not shrews). Since I am not engaged in a research project that requires permanent marking, I do not ear tag. However, we do use non-toxic markers to make an X on the belly. It does not last very long, but it works for a quick mark-recapture study.

Male with enlarged testes and large distance from the
anus to the genital
Naples, NY (7/14)


  1. Interesting with the marking... I've missed your blog entries!

  2. Love that the chipmunks can be "very bitey!"

  3. Makes me think I should by a good trail camera and see what turns up.

  4. This reminds me of the star bellied sneeches. I wonder if that mouse was ostracized by his comrades or king for the day for having a blue X.

  5. Wonderful piece of information!!

  6. Very informative reminds me of my college days doing rodent surveys for a biology class.


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