Bat Gear

Section 1: Harp Traps

This section of the web page is dedicated to talking about our gear, especially the kind we build in our basements.

Today's installment is dedicated to the harp trap. The harp trap is a piece of gear used to catch free flying bats, generally at some sort of constriction (a cave or mine entrance, a narrow point in a forest trail flyway). Harp traps have several advantages over mistnets, but have some limitations too - mainly the surface area of the trap.

Harp traps are also known as Tuttle Traps, after bat researcher Merlin Tuttle who brought harp trap design and use to new levels. Back in the day harp traps were designated as single or double bank (# of rows of monofiliment). Single bank traps are seldom used anymore, so the single/double designation has fallen into disuse.

The pictured harp trap and all harp traps used on this project were constructed by John Chenger of Bat Conservation and Management. Many of the design elements are his improvements on designs used by previous generations of bat researchers.

The anatomy of a harp trap.

The fully assembled trap, catch box and holding cage.

The "Harp" part of the trap. Note that there are two layers of monofiliment line running from the top of the trap to the lower bar. Most bats see/echolocate the monofiliment, but think they can turn sideways and pass through it. They pass through the first bank of lines, cannot maintain sideways flight for long, and bounce unhurt off the second bank. Some bats don't notice the traps and hit the first bank, so the catch box must be designed to capture bats coming down on both the inside and outside of the trap frame.
   

The catch box is designed to catch bats and funnel them down into a holding cage. The catch box must be designed so bats cannot jump/fly/crawl out and also must be accessible so researchers can get a hand in to remove bats that do not crawl down into the holding cage.

Plexiglas and bungie cords!

Bats cannot get a grip on Plexiglas, so it is used to block bats from climbing out and to slide them down into the catch box (after the monofiliment has tripped them up).
Small bungie cords are used to hold removable Plexiglas slides at proper angle and keep bats from escaping between the wood and Plexiglas.

Holding cage lid. Velcro and staples.

Bats climb down from the catch box through the PVC tube and into the holding cage. Most bats remain in the holding cage, the PVC entry way is too slick for them to climb back up. A few do jump back up into the catch box, but they still don't get out.

The two access ports are used to remove the bats from the holding cage.

Yup you guessed it, it's the golden gate bridge.

No, this is actually the view a bat who hit the first bank of monofiliment and is heading towards the cage would have.

He'll slide down the Plexiglas into the catch box and run around for a while. Then the bat will climb/drop down the hole into the holding cage (the white crescent in the bottom center is the portal into the holding cage). There he/she will await identification.

Front view of all three pieces in fully assembled glory.

 

By far the three largest advantages of harp traps are their ease of use, greater capture rates, and low stress/injury rates inflicted on the bats captured.

Harp traps do not need the level of monitoring that mistnets require, in a busy swarm checking the traps every 1/2 hour is sufficient, in slow periods/small sites every two hours is more than frequent enough. In heavy activity areas it takes quite a bit of experience to get bats out of mistnets without damaging the net or the captured bats.

Bats see/echolocate mistnets and harp traps a large percentage of the time. Harp traps used in the same setup as mistnets catch more bats. They seem to think they can get through harp traps, while mistnets must look solid enough that a greater % bother to evade them.

Bat injury and mortality in harp traps is extremely low, bats are never entangled, the whole process is fairly low stress. Mistnetting injuries of bats (though not a frequent occurrence) happen much more often.

Harp traps of a larger size are made for trapping trails. We have been working on even larger harp traps for hauling up into the canopy and trapping for species that prefer to forage and travel in that region of the forest profile.

Tom Malabad and the fully assembled trap.

The limits/drawbacks/dangers of harp traps..... Predation, if your trap is low to the ground, you must remember that you have created a nice bat buffet for the next carnivore/omnivore to pass by. Raccoons, cats, coyotes, and more would love to eat your bat catch. Stay close to setups vulnerable to predation.

Complacency - In very high catch areas you must keep track of how many bats you are catching. Bats can pile up on top of each other to the point of suffocating those at the bottom of the pile. This is a very rare situation, bats are seldom caught in numbers this high. When trapping areas where large #'s of bats are possible, use more traps and change holding cages more frequently.

Size - Harp traps do not cover the area that a mistnet can, and because the catch box and holding cage are under the frame of the trap, setting up close to the surface of water is not feasible.