Methods for the Study of Bats


Bat roost selection varies considerably; they will roost in caves, mines, occupied and unoccupied buildings, trees, under leaves, under bark, under rocks and in cliff crevices (Kunz, 1982). Usually, roosts occupied by colonial species are easier to find because of the number of bats present and because some of these species exhibit high fidelity to their roosts. This is especially the case for species that form nursery colonies, as roosts that provide adequate climatic environment for fetal and neonatal development are at a premium. Most researchers conducting long-term monitoring of wild bat colonies identify a permanent roost like a cave or a building, which they then visit regularly. The composition of the colony varies from year to year as bats are very vagile, likely moving repeatedly among roosts, but the presence of the species is consistent over time. An excellent way of finding areas that are used by bats is to listen for vocalizations or look for evidence of foraging (such as guano or dropped fruit). With microchiroptera, researchers can listen for echolocation calls with an ultrasonic detector. These ''bat detectors'' slow down the ultrasonic calls emitted by bats as they fly, making them audible to the human ear. High echolocation activity may indicate the area is a good foraging site or a roost is nearby. Furthermore, specific patterns of echolocation frequencies have been used to identify the species of bats found in an area.

There are several techniques for capturing bats, and success with any requires knowledge of the roosting and foraging behavior of bats, as well as their emergence and dispersal patterns (Kunz and Kurta, 1988). Regardless of technique, three critical pieces of equipment for capturing bats are a light source, gloves, and ''bat bags.'' Because bats are nocturnal, most capture sessions take place at dusk or at night when they emerge for foraging. Headlamps with halogen bulbs and battery packs are ideal for this work as they are reliable and keep a researcher's hands free for handling bats and manipulating equipment. Leather gloves should be used when handling bats to prevent bites and reduce stress to the animal. Lightweight leather gloves (e.g., baseball batter gloves) can be used for juveniles and smaller species, but heavier leather will be necessary for larger and more aggressive species, especially vampire bats and the large megachiroptera. The gloves should be soft and pliable to allow for improved dexterity and loose-fitting because sometimes it helps to let a bat chew on part of a glove during handling. ''Bat bags'' are simple drawstring bags made of muslin or nylon mesh that are used to temporarily hold bats after capture. We typically carry 50-100 bags for a trapping session so that each bat captured can be kept individually in a bag. However, this might not be practical if large numbers of bats are captured, and Kunz and Kurta (1988) describe several alternative temporary holding devices.

The most commonly used devices for capturing bats are mist nets and harp traps. It is possible to capture bats within a roost using hand nets, but this can be very disruptive and cause bats to abandon the roost. Mist nets are practical because they are lightweight, compact, easily erected, and commercially available in several lengths.

They can be erected on the ground or raised up into tree canopies, thereby targeting bats that fly at different heights. The preferred mist nets for capturing bats have four shelves formed by mesh made of 50 or 70 denier/2 ply nylon (Kunz and Kurta, 1988) that is tiered with shelving cords. The nets should be erected on 10-ft poles that are sunk into the ground and/or secured with rope. While lightweight aluminum mist net poles are commercially available, bamboo, small trees, electrical conduit and paint poles are also effective. Unable to detect the net, bats fly into the mesh and drop into a pocket formed by the netting and the shelving cords. There they usually become entangled as they try to escape the mesh. Researchers should continually monitor the mist net so that trapped bats can be removed as quickly as possible because the longer they remain in the net the more difficult it becomes to disentangle them. Removing a bat from a mist net takes some practice. It should be done from the side of the net the bat entered, and one should take particular care with wings as they can become damaged if handled roughly.

Harp traps are often used to capture bats that elude mist nets. Harp traps consist of one or two large frames holding banks of fine wires or monofilament fishing line that run vertically along the length of the frame. A large bag attached to the base of the trap serves to collect falling bats that have lost momentum after flying into the bank of filaments. Current harp trap designs are collapsible and portable and have the advantage that they do not require constant attendance as the bats do not become entangled in the filaments but are held in the trap bag. It should be noted, however, that bats in the trap bag are susceptible to predators and aggressive interspecies interactions, so bats should not be left in the trap bag for too long (Kunz and Kurta, 1988).

Placement of a mist net or harp trap will influence the capture success. The most successful locations can be identified by sighting flying bats or listening for vocalizations and are usually near roosts, near sources of water such as streams and lakes, and along clearings used as flyways. Other factors that may influence capture success are amount of moonlight, rain, wind, the visual and acoustic resolution and flight behavior of the bats (Kunz and Kurta, 1988). A few cautionary notes regarding netting or trapping bats at the entrance or in large roosts such as caves, mines or attics: (a) It is not uncommon to catch large numbers of bats at one time as they emerge together from a roost. This can damage the net and distress both bats and handlers as the bats become increasingly tangled in the mesh. (b) Bats may abandon a site where they are caught and handled so it is recommended to capture bats away from roosts and keep handling to a minimum (Barclay and Bell, 1988; Kunz and Kurta, 1988).

If a study requires multiple observations on the same individual, markings will be necessary. Several methods for marking bats have been use, and the choice of method will depend on the species of bat, how long the marking must last, how visible the marking needs to be, and how often the marking can be checked or renewed. It is also very important to ensure that the object used to mark a bat weighs less than 5% of the bat's body mass as the excess weight will impact flight.

Short-term marking of bats can be achieved by clipping patches of fur down to the skin, which can take up to four months to grow back if the bat is not molting. Obviously, this technique requires close handling of the bat for identification and should not be used during hibernation as the loss of fur can interfere with a bat's thermoregulation. Alternatively, livestock markers can be used to identify individuals. The nontoxic, lead-free paint of these markers adheres well to fur, comes in a variety of colors, and is safer to use than nail polish and dyes. In our experience, a colorful spot drawn between a bat's shoulder blades will last several weeks, depending on how often the individual grooms.

Long-term marking is most commonly done by wing banding. Metallic, aluminum or plastic bands are placed on the forearm of a bat, and identification is based on a unique number and/or a unique combination of colors. Reflective colored tape may be applied to the bands to permit identification of individuals during flight and while roosting. Wing bands should be used with caution as they can cause injury and infection. The band should be loose enough to slide freely along the forearm but tight enough to not slide onto the wrist and elbow joints, which can result in severe injury and immobility (Barclay and Bell, 1988). The opening of plastic split bands should be filed down to widen the gap and to smooth the edges that touch the wing as they will otherwise tear the thin wing membrane (Lollar and Schmidt-French, 1998) causing severe wounds likely to become infected. Some bats may chew at bands to the point of self-mutilation. In such instances, researchers should consider alternative methods. For example, neck collars made of bead-clasp chain or rachet-style plastic ties have been successfully used to mark some bats (Barclay and Bell, 1988; Issac et al., 2003). It is important that the collar fit properly. Wounds and infection can result from tight collars, and small bats can trap their wrist under a loose collar. We do not recommend the use of either wing bands or neck collars for juveniles that have not yet achieved adult size as the markers can interfere with proper development.

Other banding techniques used for bats include radiotransmitters and chemiluminescent light tags which are glued to a bat's back, skin tattoos, and radioactive tags. We refer the reader to Barclay and Bell (1988) for more details, advantages and disadvantages of these techniques. Toe-clipping, ear-notching, or ear-tagging, which are routinely used to mark other mammals, should not be used with bats because toes are essential for perching and grooming and ears are critical for echoloca-tion in microchiroptera (Barclay and Bell, 1988; Lollar and Schmidt-French, 1998).

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