EVALUATION OF THE EFFICACY OF VARIOUS DEER EXCLUSION DEVICES AND DETERRENT TECHNIQUES FOR USE AT AIRPORTS
6. Deer Exclusion Techniques
a. Electric Fencing
Electric fences have been developed since the 1930s to contain livestock or exclude wildlife. Most designs of electric fences developed to control deer have evolved from livestock fence designs and have been improved over time. Electric fences are often promoted as economical alternatives to more expensive fences because they require fewer posts and use less wire than steel chain-link and high-tensile fencing. The electric fence does not function as a physical barrier fence, but rather is a psychological fence. One or more (up to 12) strands of wire are used to administer an electric shock to animals when they attempt to crawl under or through it. The wires can be baited with peanut butter to encourage deer to touch the fence with the sensitive nose and mouth area in order to train them to avoid it. Baiting is done when the fence is newly constructed and should be reapplied once per month to maintain the effectiveness of the training. The heights of electric fences (from 0.75 m to 1.5 m high) are generally lower than the height of conventional fences; they are designed with a spacing that encourages deer to attempt to crawl through the fence rather than jump over it. Electric fences use high voltage, low-impedance fence chargers to produce pulsed charges of 4,500 to 5,000 volts. The pulsing is intended to administer repeated shocks, which appear to be a better deterrent than a continuous charge (Fitzwater 1972) and to prevent death of animals by "freezing" animals on the wire, which often results with constant charges.
Chargers are powered either by main electric power supply (120 or 240 VAC models) or by 12 VDC rechargeable storage batteries that are each charged by a solar panel. Chargers are rated by joules of pulsed output energy and by the distance of wire that can be charged while overgrown with weeds. A wide variety of fence chargers, batteries and solar panels are available and they are described with each type of electric fence type below. Electric fences become ineffective if the current is intermittent or cut off. Common sources of problems are short-circuits caused by weed growth or snowdrifts contacting the fence (Porter 1983; Craven and Hygnstrom 1994) and lightning damage. Weed growth on the fence wires should be controlled by mowing the grass around and under the fence. For long perimeter fences at airports, monitoring of continuity using sophisticated electric controllers ensures that any portion of the fence that is damaged or short-circuited can be quickly repaired before a breach of the fence is made. When applicable, snowdrifts can be reduced by using snow fencing. Many electric fence operators choose to turn off the current to the snow-covered wires. This reduces the effectiveness of the fence, because deer can then crawl under the fence in areas where the snow is less deep. Lightning damage to charging equipment and insulators can be reduced, but not completely eliminated, by using fuses, surge protectors, lightning choke coils, and lightning arrestors that are recommended by electric fence suppliers, and by good electrical grounding with ground rods.
Another problem that occurs in regions with dry soil during summer months is that the soil becomes so dry that it cannot conduct the current from the negative pole of the charger to the feet of the deer and the shock becomes ineffective. Using fences designed for dry ground with bipolar wires and better grounding may reduce problems from dry soil. Therefore, proper design and installation according to manufacturer's specifications and a vigilant maintenance program are needed to ensure an effective electric fence.
Under some circumstances, such as during periods of local deer overpopulation and during the rutting season, some deer may have a very high motivation to cross electric fences. Some persistent male deer that were experienced with electric fences crawled back and forth under electric fences after learning to use their hair to insulate against shock (Lynwood Williamson, National Zoological Park, Washington, DC, pers. comm.). Most electric fences were originally designed to reduce deer depredation on high-value crops and orchards; the goal was to reduce deer damage to a tolerable level rather than to completely exclude deer. Therefore, some passage across the fence was tolerable. Gates in the fence are electrified to prevent them from becoming points of access. The amount of maintenance required to ensure effectiveness of the fences is high.
Description. — Livestock electric fences are designed to contain livestock and restrict movements of predators, particularly coyotes (Canis latrans). Howard (1991) compared the two common types of agricultural electric fence:
Biological Basis. — The livestock fence design relies on the electric shock to deter livestock and exclude their predators. This type of fence is not designed to restrict deer movements.
Literature. — An evaluation by Howard (1991) of two types of livestock electric fence designs to determine if mule deer (Odocoileus hemionus) movements were affected by the fences found that both adult and immature mule deer were capable of crossing each design during a 4-year study. The study compared the number of deer crossing the fences to the number of adult deer encountering them. The net-wire fence with 2 electric outrigger wires permitted 214 deer crossings during 503 encounters (42.5%) and the 7-strand fence permitted 45 deer to cross during 346 encounters (13.0%). The findings also indicated that both designs were acceptable means of containing livestock and excluding coyotes. The 7-strand smooth wire electric fence was found to be a major barrier to mule deer migration. Howard (1991) indicated that the fence height was the factor restricting movement across it and that taller fences with 9, 10 and 12 strands would likely present a greater barrier to deer.
Recommendation. — The 1.2-m net-wire fence with 2 electric outrigger wires is not recommended for deer exclusion. The 7-strand smooth wire fence is recommended.
Literature Reviewed. — Howard 1991.
Many designs of electric fences have been developed to protect crops from depredation by deer. This diversity has developed from the degree of protection that is required and the value of the crop lost to depredation. Simple designs are intended to reduce depredation of crops by deer and to be assembled with simple skills for use during snow-free periods. More permanent designs have been developed to provide more protection (discussed in Permanent Electric Deer Fences), but at a higher cost than simple designs and generally require a fencing contractor for installation. Gates are generally made of the same material as the fence, and have gate handles. Temporary fences of simple design include the Peanut Butter Fence and the Polytape Fence.
Description. — The Peanut Butter Fence is a simple, temporary fence designed to reduce deer depredation of small gardens, nurseries, tree seedling plantations and orchards up to 1.6 ha in size. A single strand of 17-gauge (0.15-cm) smooth wire is attached at a height of 0.76 m above the ground using 22 kg of tension to 1.2-m tall wooden corner posts and supported at 14-m intervals with 1.2-m fibreglass rods (Figure 1). A mixture of peanut butter and vegetable oil with a consistency of very thick paint is applied with a roller. Aluminum foil flags (strips) are attached at 1-m intervals by wrapping them around the wire and securing them with cloth adhesive tape in areas of high deer traffic to increase the deer's curiosity of the fence. The wire is connected to the positive (+) post of a well-grounded livestock fence charger. This design is a low cost, temporary fence that requires simple construction skills to make. Craven and Hygnstrom (1994) describe the design and construction methods. Cost in 1994, excluding labour, was estimated to be $0.40 /m, excluding the fence charger. Fence chargers for this type of fence, including solar panel cost about $500 to charge 12 km of wire. A modified version of this type of fence that uses high-tensile steel wire was found to be effective in reducing deer depredation to young apple trees in orchards (Porter 1983). The effectiveness of this modification in areas of high deer densities is unknown.
Biological Basis. — Deer are attracted by the peanut butter and the foil strips, which encourage them to make nose-to-fence contact. After being shocked, deer learn to avoid fenced areas. The foil also aids in increasing the visibility of the fence to deer thereby providing some effect as a barrier to movement.
Literature. — In a comparison of various inexpensive electric deer fences, Hygnstrom and Craven (1988) found the peanut butter fence to be cost-effective in reducing crop depredation by deer during snow-free periods. Its effectiveness is limited by its low height, low durability and reduced effectiveness during some snow conditions. No long-term evaluation of this type of fence has been reported. The durability can be improved by using high-tensile wire, but at a higher cost. Effectiveness would likely be improved by using taller posts and adding one or more wires. The fence should be checked weekly for damage by deer and grounding by vegetation or snow. Craven and Hygnstrom (1994) describe the design and construction methods. This design was considered cost effective at reducing depredation of cornfields by Hygnstrom and Craven (1988), but it is of very limited use during periods of deep snow.
Recommendation. — Recommendation for deer exclusion at airports is limited due to lack of testing of effectiveness in winter conditions.
Literature Reviewed. — Porter 1983; Hygnstrom and Craven 1988; Craven and Hygnstrom 1994.
Description. — The Polytape Fence is similar to the peanut butter fence in design and use, but it uses a 1.0-cm wide, brightly-coloured polyethylene ribbon that is interwoven with fine stainless steel wires to carry the electric current. It may be constructed with one ribbon 0.6 m above the ground (Hygnstrom and Craven 1988) or with another ribbon strung along the top of the wooden posts (Figure 2). The ribbons are connected to the positive (+) post of a grounded fence charger. Peanut butter may also be applied to aid in training the deer. Craven and Hygnstrom (1994) describe the design and construction methods. Cost in 1994 was estimated to be $0.40 /m. excluding the fence charger. Fence chargers for this type of fence, including solar panel, cost about $500 to charge 12 km of wire.
Biological Basis. — Deer receive electric shocks through nose-to-fence contact. After being shocked, deer learn to avoid fenced areas. The bright ribbon improves the visibility of the fence to deer thereby providing some effect as a barrier to movement.
Literature. — In a comparison of various inexpensive electric deer fences, Hygnstrom and Craven (1988) found the single ribbon fence to be cost-effective in reducing crop depredation by deer during snow-free periods. Its effectiveness is limited by its low height, low durability and reduced effectiveness with snow. No long-term evaluation of this type of fence is known to be documented. The fence should be checked weekly for damage by deer and grounding by vegetation. This design was judged to be cost effective at reducing depredation of cornfields, but it was of very limited use when snow accumulations caused short-circuits by grounding the polytape (Hygnstrom and Craven 1988).
Recommendation. — Recommendation for deer exclusion at airports is limited due to lack of testing of effectiveness in winter conditions.
Literature Reviewed. — Hygnstrom and Craven 1988; Craven and Hygnstrom 1994.
Five permanent electric fence designs have been used to control deer. They include the Outrigger Fence, the ElectroBraid™ Fence and three high-tensile fence designs: the Offset or Double Fence, the Overhanging or Sloped Fence, and the Vertical Fence. The ElectroBraid™ Fence is an electric fence that uses polyester braided rope that has copper wire braided into the cord. It was developed in Canada for horse ranching purposes. High-tensile electric fencing has been developed from stock fences for controlling sheep, cattle and horses in New Zealand and Australia. It is claimed to provide year-round protection of crops and trees from deer depredation. On-going maintenance is required to prevent short-circuiting by herbaceous vegetation and deep snow. Extra care is required with installation of high-tension electric fences, as high-tension wires place much higher loads on posts than low-tension designs. Therefore, the end and corner posts must be properly constructed to manufacturers' specifications. High-tensile wires should be checked for tension during early summer and late fall to adjust for changes in tension due to seasonal temperature changes. A variety of electrified gates are available for use with high-tension fences. Most gates are made from galvanized steel pipe and electrified wires strung across with insulators (Figure 3). With regular maintenance, high-tensile electric fences and ElectroBraid™ fences are expected to last 20 to 30 years. Three types of high-tensile fences are described and evaluated below.
Description. — The Outrigger Fence is a dated design and has been replaced by high-tensile electric fences. An outrigger fence design described by Tierson (1969) consisted of 2.4-m long, flanged steel posts with five copper-clad steel wires, attached with wooden pole brackets with glass insulators. The bottom three wires were spaced 40.5 cm apart with the lowest one about 20 cm from the ground; the two upper wires were spaced 50.8 cm apart. The height of the top wire was 2 m above ground. The wires were electrified with an electric fence controller.
Biological Basis. — Deer receive electric shocks through nose-to-fence contact. After being shocked, deer learn to avoid fenced areas.
Literature. — Tierson (1969) reported difficulties with the outrigger fence. The fence was problematic due to high maintenance requirements and inconsistent deer responses. Running deer usually penetrated the fence. This type of fence was not considered to be worth the cost of the fence due to its poor performance. Scott and Townsend (1985) found from a survey of eight farmers that used the outrigger fence to protect fruit trees, Christmas trees or nursery crops that all farmers reported deer damage within the fenced areas. Other fence designs fared better. 46% (25) of farmers with other types of electric fences (not specified) reported crop damage and 59% (10) of farmers using high woven wire fences (2.4 to 3 m high) reported crop damage.
Recommendation. — Outrigger Fencing is not recommended for deer exclusion at airports due to ineffectiveness and poor durability.
Literature Reviewed. — Tierson 1969; Scott and Townsend 1985.
Figure 4. An ElectroBraid™ electric deer fence design.
Description. — The ElectroBraid™ Fence is a recent Canadian invention that was developed for use in horse ranching and for deer control. It uses polyester-braided rope that has copper wire braided into the cord. Seven to nine strands of this rope, 15 to 30 cm apart, are strung horizontally along fiberglass or wooden posts using up to 90 kg of tension (Figure 4). The strands are attached to the positive (+) post of a grounded fence charger producing 4,000 to 5,000 VDC. Some designs attach alternating wires to the positive post and attach the remaining wires to the ground to improve winter performance when snow reduces grounding of the animal's feet. Short-circuiting can be caused by interference by vegetation and snow cover. Regular cutting or use of herbicides should be sufficient to manage herbaceous plant growth along the fence.
Biological Basis. — The vertical high-tensile fence conditions deer to associate electric shocks with attempts to penetrate the fence through nose-to-fence contact with the wires. The high visibility of the ElectroBraid™ cord reduces accidental contact by deer. Taller designs provide more of a barrier to jumping deer. After being shocked, deer learn to avoid fenced areas.
Literature. — A six-month evaluation of an ElectroBraid™ electric fence installation at Little Rock AFB has been judged to be a success (Van Noord 2000). The 1.8-m high vertical electric fence design used nine strands of ElectroBraid™ cord spaced 22.8 cm apart (the bottom three cords were spaced 15.2 cm apart to deter small animals). The fence was installed in September of 1999 and has reduced deer sightings on the airfield from an average of 19 deer per night to one per night and 80% of nights have been deer-free. Most deer that were spotted on the field were traced to entering through an open gate. Deer that had jumped the fence were running from hunters. The fence 9,266-m perimeter installation cost of $82,200 was significantly lower than the cost of a conventional 2.44-m chain-link fence with three barbed wires on angle extensions ($400,000) with an equivalent effectiveness. Snow accumulation should be ploughed away from the fence to maintain winter effectiveness.
Recommendation. — Recommended for deer exclusion at airports due to effective exclusion of deer.
Literature Reviewed. — Van Noord 2000.
Figure 5. The offset or double electric deer fence (from Craven and Hygnstrom 1994).
Description. — The Offset or Double Fence is a variation of the New Hampshire Fence that was specified by the New Hampshire Fish and Game Department to protect gardens, nurseries and tree plantations from deer depredation. The design uses three high-tensile electric wires supported by two posts (Palmer et al. 1985; Hall 1996), or one post with cantilevered crosspieces (Fitzwater 1972; Craven and Hygnstrom 1994) so that the wires are arranged in an offset configuration (Figure 5). Two wires are positioned one above the other (one about 38 cm above the ground and the other about 100 cm above the ground). The third wire is positioned about 100-130 cm from the first two on the side excluded from deer and about 75 cm above the ground. The wires are attached to the positive (+) post of a grounded fence charger. Craven and Hygnstrom (1994) indicated that more wires could be added to maintain effectiveness in areas with high deer populations, but the results of this type of modification have not been evaluated. Cost of this type of fence is expected to be lower than vertical high-tensile fences due to the use of fewer wires and shorter posts. The cost for installations as large as airports was not available.
Biological Basis. — This type of electric fence uses a three-dimensional arrangement of wires to discourage deer from jumping the fence and an electric shock to repel the deer.
Literature. — A review of fence types by Fitzwater (1972) indicated that the Offset Fence costs less than a 2.4-m wire mesh fence, but requires more area to give the fence its three-dimensional shape. Palmer et al. (1985) judged this design to be effective at reducing depredation of crops. However, it was less effective in excluding deer during 30-day experimental periods than a 1.47 m high "Penn State Vertical Electric Deer Fence", which allowed no deer to cross. However, electric fences are of limited use during periods when snow accumulation is high enough to short circuit the lowest wire.
Recommendation. — Not recommended for deer exclusion at airports, as tall, vertical high-tensile fences are more effective.
Literature Reviewed. — Fitzwater 1972; Palmer et al. 1985; Hygnstrom and Craven, 1988; Craven and Hygnstrom 1994.
Figure 6. The slanted seven-wire electric deer fence (from Craven and Hygnstrom 1994).
Description. — A variety of Slanted or Overhanging Fence designs have been developed to exclude deer from moderate-to-large orchards, nurseries and fields with other high valued crops (Selders et al. 1981; Craven and Hygnstrom 1994). One common design uses seven high-tensile wires parallel to each other on a steep angle relative to the ground using support wires, boards and battens (Figure 6). The wires are positioned 30 cm apart, arranged with a slope toward the area confined by the fence. The bottom wire is positioned 25 cm from the ground. The top, third, fifth and bottom wires are connected to the positive (+) post of a grounded, low-impedance fence charger. The second, fourth and sixth wires are connected to ground. The area up to 4 m along the outside of the fence is cleared and maintained to ensure visibility of the fence. This type of fence covers 2.44 m of horizontal distance, but because of the angle of the fence it is only 1.2 m high. A 1.5-m high sloped fence design with seven strands of high-tensile wire (O'Dell 1997) was successful in obtaining "100% deer control" in a 4.8-ha strawberry farm in Virginia for 3 years. Bonwell (1983) indicated that this fence design produces a "good physical and psychological barrier". The fence should be checked weekly for damage by deer and grounding by vegetation. Cost of this type of fence is expected to be lower than vertical high-tensile fences due to the use of fewer wires and shorter posts. The cost for installations as large as airports was not available.
Biological Basis. — This type of electric fence uses the sloped arrangement of wires to discourage jumping and encourage deer to try to crawl underneath the fence whereupon the electric shock that repels the animal is received. The electric wires are initially baited to encourage conditioning of the deer to the fence.
Literature. — A comparison of various inexpensive electric deer fences (Hygnstrom and Craven 1988) found the electric sloped fence to be cost-effective in reducing crop depredation of high value crops and orchards by deer during snow-free periods. Its effectiveness is limited by its low height, and limited effectiveness with snow. The success of the 1.5-m high design at a Virginia strawberry farm (O'Dell 1997) for 3 years may partially be a result of mild winters found there. Snow accumulation may not have risen to the height of the lowest electric wire. Palmer et al. (1985) tested a 1.2-m high non-electric version of the Sloped Fence design; they found that deer were capable of jumping over the height and breadth of the fence.
Recommendation. — Recommendation for deer exclusion at airports is dependent upon successful long-term testing in winter conditions.
Literature Reviewed. — Selders et al. 1981; Bonwell 1983; Palmer et al. 1985; Hygnstrom and Craven 1988; Craven and Hygnstrom 1994.
Figure 7. The seven-wire vertical electric deer fence (from Craven and Hygnstrom 1994).
Figure 8. Electrical and grounding system for high-tensile fences, featuring wires with alternating polarity for improved effectiveness in winter (from Craven and Hygnstrom 1994).
Figure 9. The Penn State vertical electric deer fence (from Palmer et al. 1985).
Description. — Vertical high-tensile fence designs have been developed over the past 20 years to protect large truck gardens, nurseries, orchards, reforestation projects and airports (Bonwell 1983; Smith and Kearley 1984; Palmer et al. 1985; Scott and Townsend 1985; Craven and Hygnstrom 1994). Some zoos use electric fencing to deter hoofed-stock and large game, such as rhinoceros and tigers, from barrier fencing to reduce damage. The vertical high-tensile fence designs vary in height, from 1.2-m to 1.8-m and in the number of electric wires, from five to seven. The general design consists of solid wooden or steel posts spaced approximately 60 m apart and strung with vertically arranged high-tensile wire on plastic insulators. Tension of between 11 to 114 kg is applied to the wires with an in-line wire strainer on each wire and tension is maintained by an indicator spring (Figure 7). The wires are attached to the positive (+) post of a grounded fence charger producing 4,000 to 5,000 VDC. Some designs attach alternating wires to the positive post and attach the remaining wires to the ground to improve winter performance when snow reduces grounding of the animal's feet (Figure 8). Short-circuiting can be caused by interference from vegetation and snow cover. Regular cutting or use of herbicides should manage herbaceous plant growth along the fence. Costs for entire installations were difficult to obtain, but they are estimated to be about $7,700/km based on conversations with wildlife workers and manufacturer documentation.
Biological Basis. — The vertical high-tensile fence conditions deer to associate electric shocks with attempts to penetrate the fence through nose-to-fence contact with the wires. Contact is sometimes facilitated by the application of attractants, such as peanut butter. Taller designs provide more of a barrier to jumping deer. After being shocked, deer learn to avoid fenced areas.
Literature. — A comparative study by Palmer et al. (1985) found that deer were fully excluded by the 1.47-m high "Penn State Vertical Electric Deer Fence" design during 30-day experimental periods at Penn State University Experimental Forest. The design used five electric wires spaced 30.5 cm apart with the bottom wire located at 25.4 cm from the ground (Figure 9). The "Penn State Vertical Electric Deer Fence" design has been improved by increasing the height to 1.8 m and increasing the number of electric wires to seven. Craven and Hygnstrom (1994) indicated that although vertical electric fences take up less ground space than three-dimensional fences, they are less likely to inhibit deer from jumping over. Joe Harding (Director of Forest Lands, Pennsylvania State University, pers. comm.) indicated that this fence design is the best choice for forest protection, because the high-tensile wire resists breakage from falling tree branches and the wires will spring back to their original position after the trees are removed. Long-term use of this design by Penn State University has indicated that, with regular maintenance, this fence design works to reduce deer depredation to an acceptable level. However, during the rutting season, penetration of this type of fence by deer can be frequent. One fence supplier (Bonita Whalen, Kiwi Fence Systems, pers. comm.) admitted that the taller electric high-tensile fences provide only 70-80 percent protection even when fully electrified and maintained. Lynwood Williamson (National Zoological Park, Washington, DC, pers. comm.) and Joe Harding (Director of Forest Lands, Pennsylvania State University, pers. comm.) confirmed this fact. Even when fully operational some deer learned to crawl through the fence without contacting the wires with their nose or mouth and use their fur as a partial insulator to the shocks.
Many regional airports in the eastern United States are in the process of removing high-tensile electric fences and replacing them with 2.4-m high chain-link fencing, topped with 3 strands of barb wire (Richard Atkinson, Airport Director, Central West Virginia Regional Airport Authority, Charleston, WV, pers. comm.; Scott Byard, Assistant Operations Manager, Tri-State Airport, Huntington WV, pers. comm.). The most important reasons for switching were maintenance issues, which resulted in poor performance of the high-tensile fence. High-tensile electric fencing is well-suited for the exclusion of deer for forest or crop protection, but it may not be suitable for excluding deer at airports if maintenance is not consistent. Electric fences have reduced effectiveness during periods of snow accumulation above the lowest electric wire due to short-circuiting by snow.
Recommendation. — Recommendation for deer exclusion at airports is limited due to incomplete exclusion of deer, high maintenance requirements, and problems caused by snow in winter.
Literature Reviewed. — Bonwell 1983; Smith and Kearley 1984; Palmer et al. 1985; Scott and Townsend 1985; Craven and Hygnstrom 1994.