TP 13029
A variety of devices and materials have been used to provide apparent or actual barriers to prevent birds from entering areas. If birds are physically excluded from an area or feature, then the efficacy of the technique is obvious. Birds will continue to be excluded as long as the physical barrier is maintained. There are other techniques that exclude by providing an apparent, rather than an actual physical, barrier to access. However, we found few research papers that reported on the efficacy of these "virtual" exclusion methods. In the following discussion, general considerations regarding physical barriers are presented first, followed by the results of studies on " virtual barrier" exclusion methods.
General Considerations Regarding Actual Physical Barriers
Description. – Actual physical barriers include devices and materials (1) to cover or surround an area (netting and fencing); and (2) to prevent perching, roosting and nesting on surfaces (Nixalite, Bird-B-Gone, Avi-Away, and fine wires strung along ledges). Bird-exclusion netting is made out of polyethylene, other synthetic materials, or cotton, and is available in a variety of mesh sizes. Nixalite and Bird-B-Gone are strips of short metal (Nixalite) or plastic (Bird-B-Gone) prongs placed along perches such as window ledges or pipes. Avi-Away is an electrical cable that can be placed along perches; a bird receives a mild shock when it lands on the cable. Taut stainless steel wires also can be run along perches to prevent birds from landing. These methods are described in greater detail in the Transport Canada "Wildlife Control Procedures Manual" (Transport Canada 1994).
Biological Basis. – If access is denied to a feature of an airport that is attractive to birds, such as a feeding, loafing, roosting, or nesting area, then the feature will not be as attractive. Birds likely will leave the immediate area and look elsewhere for these features.
Literature. – Netting is sometimes used to prevent songbirds from feeding on high-value crops such as cherries, blueberries, and grapes (Grun 1978; Twedt 1980; Biber and Meylan 1984; Cocci 1986). Netting is also occasionally used in attempts to keep birds out of airport facilities, buildings, or other locations (LGL Ltd. 1987; Skira and Wapstra 1990). Netting is widely used to deter fish-eating birds from aquaculture facilities on land and offshore (EIFAC 1988; Kevan 1992).
Fences made out of poultry wire (or cable), plastic (Vexar Fencing), netting, and electrical wire, have all been used to deter birds from fish culturing facilities (Mott 1978; Meyer 1981; Ueckermann et al. 1981). Fencing has also been used to keep pigeons from roosting on ledges of buildings, and electrified fences have been effective in some situations for deterring both birds and mammals where regular fences were ineffective (see Koski and Richardson 1976).
Evaluation. – Because of high costs, it would not be practical to use these exclusion products where birds must be kept out of large areas. These devices would be most useful in situations where the risk was confined, or for deterring birds from landing at specific sites such as nest sites. Other deterrent devices, such as pyrotechnics, may enhance deterrent efforts when netting is being used (EIFAC 1988).
Constructing fences may be effective for excluding birds from airfields if birds are flightless (i.e. moulting or brood rearing adult, or pre-fledging young, geese and ducks) and if the area of concern was relatively small. Fences and nets might be useful in keeping moulting waterfowl or broods from runways, or in combination with trapping (see below) to move flightless birds from the area. Fences generally would not be practical in many airport situations because the areas involved would be too large to fence, and because most birds would be able to fly over them.
Advantages
- These materials are readily available.
- When properly installed and maintained, actual physical barriers exclude birds permanently from the treated area.
Disadvantages
- None of these techniques is practical for large areas.
Recommendation. – Recommended for site-specific problems.
Literature Reviewed. – Barlow and Bock 1984; Biber and Meylan 1984; Blokpoel and Tessier 1987; Cocci 1986; Devenport 1990; Dolbeer et al. 1988; EIFAC 1988; Galbraith 1992; Glahn et al. 1991; Grun 1978; Kevan 1992; Koski and Richardson 1976; Littauer 1990b; Lucid and Slack 1980; Meyer 1981; Moerbeek et al. 1987; Mott 1978; NCC 1989; Salmon and Conte 1981; Salmon et al. 1986; Skira and Wapstra 1990; Spear 1966; Twedt 1980; Ueckermann et al. 1981; Whittington 1988.
Description. – A grid, or series of parallel lines, of fine wire or monofilament line is strung on a level plane over the area from which birds are to be excluded. The spacing of the lines varies from approximately 1.5 to 12 m, depending on the species and feature to be treated. At airports, overhead lines can be strung over ponds and puddles, for example. Sometimes wires/lines around the sides of the area are required as well.
Biological Basis. – The reason or reasons for the repelling effect of overhead lines or wires are not well understood. Wires that are closely spaced, e.g. 1 m or less, may come close to forming an actual physical barrier. However, birds can be deterred by wires whose spacing is much greater than the dimensions of the bird. The element of surprise seems to be important - the unexpected encounter of a bird with a thin, difficult-to-see line has a startling effect.
Literature. – As early as 1936, overhead wires or lines were recommended as a method of deterring waterbirds from reservoirs and fishponds (McAtee and Piper 1936). In the past two decades, widely-spaced overhead wires have been used to reduce the numbers of gulls attracted to landfill sites, reservoirs, pools, picnic areas, and beaches in the U.S.A. and Canada. Wire spacing has varied widely, from less than 1 m to as much as 25 m. Even lines that are very widely spaced relative to the wingspan of gulls seem to have some deterrent effect. The gulls are reluctant to fly down between the lines. In a few cases, systematic counts of gulls and other birds have been made in the presence and absence of the wires. These studies have shown that the deterrent effect on gulls is quite pronounced (e.g. Blokpoel and Tessier 1984; Forsythe and Austin 1984; McLaren et al. 1984), even when deterring gulls from their own nests (Blokpoel and Tessier 1983; Belant and Ickes 1996). Areas as large as 220 acres have been covered by wires in order to deter gulls from landfill sites (Dolbeer et al. 1988).
Other types of sites where overhead lines or wires have been applied include fish-rearing facilities (Ostergaard 1981; Salmon and Conte 1981; Barlow and Bock 1984; Salmon et al. 1986; Moerbeek et al. 1987), airports (Blokpoel and Tessier 1987), fruit crops (Steinegger et al. 1991; Knight 1988), and backyard feeding stations (Agüero et al. 1991; Kessler et al. 1991). The effectiveness of overhead wires or lines varies widely among species and circumstances. However, some deterrent effect has been shown for a variety of waterbirds, including various gulls, ducks, geese and cormorants (Pochop et al. 1990).
The required line spacing is highly variable, depending upon the species of bird being deterred, the activity of the birds, and the structure or crop that needs protection. To repel gulls from a fish hatchery or nesting colony, the lines must be close together, whereas at a landfill site they can be 3-12 m apart (McLaren et al. 1984; Pochop et al. 1990).
Evaluation. – The principal disadvantages to overhead wires and lines are cost and poor mobility.
There are several advantages also:
- Overhead lines do not rely on the skill and motivation of bird controllers.
- They do not require continuous attention.
- Gulls at least do not seem to habituate to overhead lines.
- Gulls that do gain access under the lines are extremely nervous and, therefore, very susceptible to active dispersal techniques.
While overhead lines/wires could not be installed over large areas at an airport without incurring considerable cost, widely-spaced lines/wires would be an effective and relatively permanent solution where a bird problem is localized.
Recommendation. – Recommended for exclusion of birds from relatively small open areas (up to a few ha).
Literature Reviewed. – Agüero et al. 1991; Amling 1980; Blokpoel and Tessier 1983, 1984, 1987; Forsythe and Austin 1984; Kessler et al. 1991; Knight 1988; Koski and Richardson 1976; McAtee and Piper 1936; McLaren et al. 1984; Ostergaard 1981; Pochop et al. 1990; Steinegger et al. 1991.
Foam has been used at some sanitary landfill sites as an alternative to earth for the daily cover material. Although quantitative data are not available, gulls that were attracted in large numbers to one landfill site seemed reluctant to walk into the foam cover (manufactured by Rusmar Foam Technology) used at that site (R. Harris, LGL Ltd., unpubl. obs.). The effectiveness of foam would also depend upon weather conditions; for example, its effectiveness would be reduced in rainy or windy weather. The applicability of foam to airport situations seems limited. It could perhaps be used to cover small areas that, for one reason or another (e.g., food source, pond/puddle), are particularly attractive to birds.
Bird Balls™ is a relatively new and promising product that has been used to exclude water birds from industrial ponds in the western United States since 1993. Birds are denied access to a pond by covering the entire water surface of the pond with these plastic (HDPE), 4-in diameter spheres. Although birds could probably land in the pond, and the balls would shift aside, the balls "hide" the water surface from the birds. Birds do not see the area as a pond. The balls are superior to netting or floating membranes in that they adjust to fluctuating water levels and snow loads, readily shift around in-water obstacles, are unaffected by all but the strongest winds (>50 mph; Mike Taber, Wildlife Control Technology, Inc., pers. comm), are very easy to install (empty the bags of loose balls into pond), and require significantly less maintenance. Bird Balls™ are initially substantially more expensive than overhead lines and netting. Current cost ranges from U.S. $0.85-3.00 per sq ft (10 balls). Wildlife Control Technology, Inc. is the sole North American distributor.
We are aware of no independent, objective studies of the efficacy of Bird Balls™. Nevertheless, the technology appears to be simple, straightforward, and sound.