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World Owl Trust - leading the World in Owl Conservation
Friday 25th July, 2014

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Mammal Trapping - A Key Tool In Conservation

As a part of our conservation work, we completed a survey of voles in rough tussocky grassland. In order to do this, we set up a grid of mammal traps to evaluate the current population numbers. This allows us to see if there is a sufficient short-tailed field vole population to sustain the Barn Owl diet, i.e. compared to other rodents, e.g. bank voles or field mice.

The traps we use are made by “Longworth” which comes in two sections. The box at the back has an inner flap to secure the trap (tunnel compartment). Some of the apparatus have a “shrew” hole at the back of the box to allow the shrews to escape (there are certain laws and regulations on trapping shrews).

The trap works when the trapdoor hatch is resting on the latch connecting to the bar at the back of the tunnel. If the bar is hit, i.e. if the animal touches the bar, the tunnel trapdoor closes, and the animal is now inside the trap. They have plenty of food and warmth to survive the night, and it ensures they have the best chance at survival once we release them back into the same location.

There are two latches in the tunnel compartment – one for keeping the trapdoor hatch locked open without the animal setting it off, and second latch to set the trap on. For the first 1 – 2 nights we put down the apparatus with the trapdoor locked, so that the animal has access to food, and in doing this, allows the animal to assume it is safe to come and go as it pleases.

We use Hay for the bedding to keep the animal warm overnight, and a good handful is needed to fill the box. Next in is the food – researchers in the past have used various kinds of bait, ranging from fresh apple slices to peanut butter spread, but we won’t use these as they are very smelly and messy (and possibly attracting other predators i.e. badgers or foxes who could disturb/damage the traps). For our survey we used a mixture of dried bird food with peanuts.

Later when we set the traps to close and trap, we add in smellier food, such as Casters (commonly used in fishing) or Cat food, this is for two reasons, so that the animal cannot resist going into the trap, and secondly, for the boxes which do not have a shrew hole, the extra protein is needed for shrews to survive the night (they use up a lot of energy).

In order for the trap to work efficiently, it is positioned on the vole runs found in the tussocky grass, usually on a worn vole path, and the trapdoor entrance is facing the recent sign of activity i.e. eaten shoots, nesting material, droppings or even a hole into the soil. All of the traps are covered with long grass, to reduce the danger of discovery from predators.

Once the trap is set to catch, we come back to the site first thing in the morning so to reduce the animals stay in the trap. On our morning’s return, the apparatus is either discovered with the trapdoor open or shut. If the trapdoor is closed, it indicates that the animal has been inside. Sometimes nothing is found even though the trapdoor is closed.

The first video shows the point where we discover who is hidden inside by taking the trap apart and collect data for our survey.

Part 1 - Taking The Trap Apart

In the Video Hilary is seen holding the Longworth trap inside a see-through plastic bag. The plastic bag is tied tightly around the box to prevent the mammal escaping. Firstly the outer latch is released to separate the two compartments. Most of the time, mammals are found hidden inside the hay box, but as seen in the video, the short-tailed field vole was found hiding in the tunnel. The vole is then released and examined in the plastic tank so we can identify its species type from the fur colouring, shape of ears, and tail length.

Part 2 - Weighing The Mammal

In this video the vole is seen inside the plastic bag. The plastic bag has been weighted beforehand so when it goes on the scales we can calculate how much the vole weights (in grams). By weighing all the mammals we can work out the average weight for the population and check that they are in a healthy weight range.

Part 3 - Releasing The Vole

Once we have gathered all of our measurements and data we then release the vole back to where it was found.

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