Citation: Babcock and Booth (2020) Anti-predator Fencing. Tern Conservation Best Practice. Produced for “Improving the conservation prospects of the priority species roseate tern throughout its range in the UK and Ireland” LIFE14 NAT/UK/000394’
Last updated: October 2020
This is a live document we update regularly. If you have comments and suggestions, please email Chantal.Macleod-Nolan@rspb.org.uk
Last updated: October 2020
This is a live document we update regularly. If you have comments and suggestions, please email Chantal.Macleod-Nolan@rspb.org.uk
| Babcock and Booth (2020) Anti-predator Fencing. Tern Conservation Best Practice. |
Key Messages
- There are two main types of fencing associated with tern colonies: fences designed to exclude mammalian predators, and fences to protect areas of breeding tern habitat from disturbance by human activities and dogs.
- The most suitable design for an anti-predator fence depends on the ecology of the species being protected (particularly how predictable the nesting area is), the predator being excluded, the physical characteristics of the site, the level of public access, budget and acceptable maintenance level.
- Anti-predator fences can be temporary or permanent, and may be barrier, electric stranded or a combination of these.
Information
It is widely recognised that predation by generalist mammalian predators can reduce productivity of ground-nesting birds, including terns, below the minimum needed to maintain a population. The species’ response to this threat is evident in the high concentration of affected species on predator-free islands and within anti-predator fences on nature reserves compared to surrounding areas. The behaviour of many tern species indicates that if a colony suffers heavy predation the breeding terns will relocate to nearby sites with suitable habitat either within the breeding season, or the following year. This pattern is sometimes seen, however, if there are no suitable alternative sites, which is increasingly the case as habitat is lost, for example to sea level rise, development and recreational disturbance, it is important that protection is provided at the existing site.
Most nature conservation organisations prefer to use non-lethal methods of predator management wherever this is practical and effective. Currently, the only non-lethal management option consistently shown to increase the productivity of vulnerable ground nesting birds is anti-predator fencing (Smith et al. 2010). It is however important to consider that although fencing is in itself a form of non-lethal management, what actions will be taken if a predator begins to regularly breach the fence or becomes trapped inside the fence.
Anti-predator fence installation should be considered at sites where terns breed in significant numbers (what is ‘significant’ will vary by species) but where productivity is low due to predation or disturbance from large ground predators. A fence will also be valuable where the ecological consequences of predation would be particularly severe, for example on Coquet Island where a fence was installed to protect the only UK regular breeding colony of roseate terns Sterna dougallii after an otter Lutra lutra began frequenting the island.
Every site is different and the decision to install a fence must be based on site specific factors, which include:
Once the decision has been made that a fence is needed, the different types of fencing and the position of the fence should be considered. The most suitable type of fence will depend on the landscape, the species being protected, the predator species present, the amount of time available to staff, vulnerability to high tides and storm surges, the budget , and these will differ by site. For colonies that tend to move around like beach-nesting little terns Sternula albifrons or least terns Sternula antillarum, or are publicly accessible places, temporary fences are usually the best option. For islands in waterbodies that are regularly used by nesting terns, barrier fencing around the individual island(s) can be the most effective and cost-effective option. In some cases it may be preferable to put a fence around the perimeter of the waterbody rather than the island.
Since every fencing situation is unique, the information provided below is not intended to act as complete guide, however, colony managers should understand the principles of anti-predator fences to make the right choice for their particular situation. For more technical information see White and Hirons (2019). Talking to fencing contractors with experience of conservation anti-predator fencing is also likely to be useful.
Fencing cannot prevent all mammalian predation at tern colonies indefinitely and it cannot exclude avian predators. Colony managers should aim to achieve a level of productivity which is likely to sustain or increase the colony (but there is no specific target figure for this as recruitment to the colony is also dependent on winter survival, immigration and emigration).
It is widely recognised that predation by generalist mammalian predators can reduce productivity of ground-nesting birds, including terns, below the minimum needed to maintain a population. The species’ response to this threat is evident in the high concentration of affected species on predator-free islands and within anti-predator fences on nature reserves compared to surrounding areas. The behaviour of many tern species indicates that if a colony suffers heavy predation the breeding terns will relocate to nearby sites with suitable habitat either within the breeding season, or the following year. This pattern is sometimes seen, however, if there are no suitable alternative sites, which is increasingly the case as habitat is lost, for example to sea level rise, development and recreational disturbance, it is important that protection is provided at the existing site.
Most nature conservation organisations prefer to use non-lethal methods of predator management wherever this is practical and effective. Currently, the only non-lethal management option consistently shown to increase the productivity of vulnerable ground nesting birds is anti-predator fencing (Smith et al. 2010). It is however important to consider that although fencing is in itself a form of non-lethal management, what actions will be taken if a predator begins to regularly breach the fence or becomes trapped inside the fence.
Anti-predator fence installation should be considered at sites where terns breed in significant numbers (what is ‘significant’ will vary by species) but where productivity is low due to predation or disturbance from large ground predators. A fence will also be valuable where the ecological consequences of predation would be particularly severe, for example on Coquet Island where a fence was installed to protect the only UK regular breeding colony of roseate terns Sterna dougallii after an otter Lutra lutra began frequenting the island.
Every site is different and the decision to install a fence must be based on site specific factors, which include:
- An understanding of the causes of low productivity (unless it is due to mammalian predators or disturbance then fencing is not the answer).
- Any factors which threaten the future of the colony (apart from predation pressure), and whether these can be managed.
- The size and topography of the area to be fenced. The fence line must not pass close to vantage points which a predator could use to jump over the fence. Ditches and gates are more challenging to fence effectively and create potential weak points.
- There must be a significant population of the species needing protection within the area to be fenced. In a large dispersed colony it may be necessary to focus on the area(s) where the population density is highest.
- Cost: money and staff time including for preparatory work, installation and future maintenance
- The fence must contain appropriate habitat with a low likelihood of flooding, with a programme in place for management to maintain this.
- Proximity of public access and the potential for disturbance and conflict, use of the area at other times of year.
Once the decision has been made that a fence is needed, the different types of fencing and the position of the fence should be considered. The most suitable type of fence will depend on the landscape, the species being protected, the predator species present, the amount of time available to staff, vulnerability to high tides and storm surges, the budget , and these will differ by site. For colonies that tend to move around like beach-nesting little terns Sternula albifrons or least terns Sternula antillarum, or are publicly accessible places, temporary fences are usually the best option. For islands in waterbodies that are regularly used by nesting terns, barrier fencing around the individual island(s) can be the most effective and cost-effective option. In some cases it may be preferable to put a fence around the perimeter of the waterbody rather than the island.
Since every fencing situation is unique, the information provided below is not intended to act as complete guide, however, colony managers should understand the principles of anti-predator fences to make the right choice for their particular situation. For more technical information see White and Hirons (2019). Talking to fencing contractors with experience of conservation anti-predator fencing is also likely to be useful.
Fencing cannot prevent all mammalian predation at tern colonies indefinitely and it cannot exclude avian predators. Colony managers should aim to achieve a level of productivity which is likely to sustain or increase the colony (but there is no specific target figure for this as recruitment to the colony is also dependent on winter survival, immigration and emigration).
Barrier Fences
Wire mesh or high tensile wire used to create a rigid netting framework, supported on metal or sometimes wooden posts, barrier fences in the UK are usually intended to exclude foxes Vulpes vulpes and sometimes badgers Meles meles, otters or hedgehogs Erinaceus europaeus. To effectively exclude a fox on land the height of a fence must be 1.8 - 2.0 m with an outward facing overhang or a ‘floppy top’ and a mesh size no greater than 70 - 80 mm (although note that cubs will still be able to pass through this), and be either buried vertically into the ground 25 - 45 cm, or with a mesh ‘skirt’ extending 30 - 60 cm out from the fence to prevent animals from digging under it. It is important to put the fence posts INSIDE the mesh to avoid providing opportunities for foxes to climb (the opposite way round to a livestock fence), and the design of strainers and gates must be carefully considered to avoid creating weak points. If well-designed and well-installed, barrier fences are less likely to fail, and have a longer lifespan than an electrified fence.
A barrier fence placed in water (if that water is wide and deep enough to require a fox to swim) does not need to be so tall, perhaps only 30 cm above the water level, however if the water level is likely to fluctuate the height must take account of this. Recycled plastic posts may be more durable than wood where changing water levels will repeatedly wet and dry the posts and lead to wood rotting, however the value of this must be weighted against the cost and the desirability of introducing plastic into the environment. A floating fence which can rise and fall with changing water levels is another way to overcome the issue of changing water levels.
A very low tech and cheap ‘floating rope’ design has been used in 3 or 4 places in Denmark for 6 - 7 years and found to be effective against foxes in sheltered waters where the water is deep enough that a fox has to swim. This is based on the idea that a swimming fox does not like to put it’s head under water. The fence is made of a minimum 8 mm thick nylon rope, with floats attached at 1m intervals. To make the set-up and dismantling of the floating fence as easy as possible the rope is made in 25 - 50 meters pieces joined together with shackles. Every 25m anchor ropes are attached to the floating rope with either shackles or carbines, connecting via anchor ropes to pieces of heavy chain which act as anchors. The key is to ensure that the anchoring system does not allow a swimming fox to push the rope below the surface. A Danish net-maker invented this system, working with the Ministry of Environment and Food of Denmark.
Wire mesh or high tensile wire used to create a rigid netting framework, supported on metal or sometimes wooden posts, barrier fences in the UK are usually intended to exclude foxes Vulpes vulpes and sometimes badgers Meles meles, otters or hedgehogs Erinaceus europaeus. To effectively exclude a fox on land the height of a fence must be 1.8 - 2.0 m with an outward facing overhang or a ‘floppy top’ and a mesh size no greater than 70 - 80 mm (although note that cubs will still be able to pass through this), and be either buried vertically into the ground 25 - 45 cm, or with a mesh ‘skirt’ extending 30 - 60 cm out from the fence to prevent animals from digging under it. It is important to put the fence posts INSIDE the mesh to avoid providing opportunities for foxes to climb (the opposite way round to a livestock fence), and the design of strainers and gates must be carefully considered to avoid creating weak points. If well-designed and well-installed, barrier fences are less likely to fail, and have a longer lifespan than an electrified fence.
A barrier fence placed in water (if that water is wide and deep enough to require a fox to swim) does not need to be so tall, perhaps only 30 cm above the water level, however if the water level is likely to fluctuate the height must take account of this. Recycled plastic posts may be more durable than wood where changing water levels will repeatedly wet and dry the posts and lead to wood rotting, however the value of this must be weighted against the cost and the desirability of introducing plastic into the environment. A floating fence which can rise and fall with changing water levels is another way to overcome the issue of changing water levels.
A very low tech and cheap ‘floating rope’ design has been used in 3 or 4 places in Denmark for 6 - 7 years and found to be effective against foxes in sheltered waters where the water is deep enough that a fox has to swim. This is based on the idea that a swimming fox does not like to put it’s head under water. The fence is made of a minimum 8 mm thick nylon rope, with floats attached at 1m intervals. To make the set-up and dismantling of the floating fence as easy as possible the rope is made in 25 - 50 meters pieces joined together with shackles. Every 25m anchor ropes are attached to the floating rope with either shackles or carbines, connecting via anchor ropes to pieces of heavy chain which act as anchors. The key is to ensure that the anchoring system does not allow a swimming fox to push the rope below the surface. A Danish net-maker invented this system, working with the Ministry of Environment and Food of Denmark.
Figure 1. On the left, the Danish net-maker and floating rope designer with a piece of floating rope, in the middle an island encircled by floating rope (notes that in this case the electrical wire intended to protect the floating rope from livestock has mistakenly been placed inside the floating rope) and on the right, an anchor chain being deployed.
Although not aimed at preventing mammal predation, on the Isle of May low chicken wire fences are placed around the tern terraces to prevent large gull species and oystercatchers Haematopus ostralegus from walking into the tern colonies to predate eggs and chicks (an action which evokes surprisingly little defensive reponse from the terns compared to an aerial attack). This barrier appears to prevent some opportunistic predation by stealth and to give the terns more time to react. Placing low chicken wire fences along the edges of visitor paths (as occurs on the Isle of May and on Farne Islands in Northumberland) can also prevent chicks running out and being at risk of being stepped on, especially in any public areas.
Electric fences
Combining a weak physical barrier and a psychological imprint to modify the behaviour of the predator (the animal associates the shock with the fence to create a psychological barrier that discourages it from touching it again) electric fences can be powered by mains electricity or a battery powered energizer attached to the wires which produces a short but unpleasant shock when the wire is touched. The low amperage and short duration (about 1/300th. of a second) results in a sharp but safe sting (although note that the defensive response of hedgehogs curling into a ball means they can be at risk of prolonged exposure).
The configuration of a stranded wire electric fence depends on the predator; a fox fence must be much higher than a fence designed to exclude badgers or otters.
Combining a weak physical barrier and a psychological imprint to modify the behaviour of the predator (the animal associates the shock with the fence to create a psychological barrier that discourages it from touching it again) electric fences can be powered by mains electricity or a battery powered energizer attached to the wires which produces a short but unpleasant shock when the wire is touched. The low amperage and short duration (about 1/300th. of a second) results in a sharp but safe sting (although note that the defensive response of hedgehogs curling into a ball means they can be at risk of prolonged exposure).
The configuration of a stranded wire electric fence depends on the predator; a fox fence must be much higher than a fence designed to exclude badgers or otters.
Figure 2. Suggested wire configurations for stranded electric fences to exclude fox and badger or otter (from White and Hirons 2019).
An electric fence system includes six components:
- A battery or mains powered energiser produces a pulsed electric current along the fence wires.
- An earthing rod driven into the ground returns the electrical pulse of energy to the energiser.
- Wood, metal, plastic or fibreglass stakes to support the fence.
- Insulators attached to the fence stakes to ensure the pulsed current is not lost to earth but maintained to provide a sharp, high voltage shock to any animal touching a live wire.
- Single or multi-strand steel wires to conduct the current. Electrified plastic netting can be used against foxes, but hedgehogs may get entangled.
- A fence tester is used to check whether the fence is providing sufficient voltage.
Electric fences can be permanent, semi-permanent or temporary/portable. Semi-permanent electric fences are usually strained wire fences comprised of a series of electrified parallel conducting (i.e. live) usually steel (galvanized plain or strand) wires alternating with non-electrified earth wires at varying heights above the ground and supported at intervals by posts. They are cheaper and have lower visual impact than permanent barrier fences but are reliant on a reliable electricity supply. They also require regular maintenance to maintain wire tension and to prevent shorting by vegetation. Vegetation around an electric fence needs to be supressed, cut or treated with herbicide to prevent shorting. Note that air with a high moisture content such as fog can also drain an electric fence, making it completely ineffective until the weather changes. When an stranded electric fence has no power it presents little barrier to predators.
Figure 4. A weed barrier to prevent vegetation shorting the fence, in this case protecting waders in NE Poland (Daniel Piec).
Temporary/portable electric fences can be either:
- Wire strand fences comprising a series of electrified parallel conducting wires at varying heights above the ground. These are usually made from Polywire (polythene twine interwoven with steel strands) which is cheaper and easier to install than galvanised steel wire but is a poorer conductor. Polywire strands can be wound onto reels for easy transport and can be supported by lightweight plastic stakes rather than wooden posts.
- Electrified netting fences consisting of a heavy-duty, polythene twine mesh in which the horizontal strands are interwoven with electrically conductive stainless-steel wire supported by vertical strands of plain polythene twine. Such fences vary in height and mesh size, depending on the manufacturer, and come in 25 or 50 m rolls fitted with spiked plastic or fibreglass supporting posts built into the fencing at regular intervals with a clip at each end to join rolls together. These fences are very easy and quick to erect and dismantle.
Figure 5. An electrified netting fence at Lady’s Island Lake Ireland to protect the roseate tern colony from American mink, foxes, badgers and pine martens (Tony Murray).
The principal advantages of temporary electric fences are flexibility and relative ease of installation. They are often used at little tern breeding sites to protect nesting areas while breeding birds are present, and they can be quickly adjusted or moved as required depending on where birds settle. They are less durable than other fences and are more easily damaged. They also require regular checks against shorting or wire sag. At sites where the same area is
fenced every year semi-permanent wooden fenceposts are often used at corners/as strainer posts to assist with keeping wires taut.
Figure 6. A temporary electrified mesh fence to deter coyote at a least tern and piping plover Charadrius melodus breeding site in Massachusetts, USA (Michael Babcock)
Combination fences
Combination fences combine barrier and electric elements. The main section of these fences is a barrier of wire mesh netting, usually high tensile steel; additional electric wires are then incorporated into the design to provide extra protection. Commonly one or two electrified wires are put above the barrier, and one offset near the bottom of the fence.
A combination fence is less adaptable, but requires less maintenance than a stranded electric fence and, if something goes wrong with the electrical part, the fence can still act as a physical barrier. The barrier element means that a combination fence is mainly suitable where the same area needs protection each breeding season and the fence can be left in place (see the Coquet Island case study). Combination fences are more expensive than semi-permanent electric fence.
Combination fences combine barrier and electric elements. The main section of these fences is a barrier of wire mesh netting, usually high tensile steel; additional electric wires are then incorporated into the design to provide extra protection. Commonly one or two electrified wires are put above the barrier, and one offset near the bottom of the fence.
A combination fence is less adaptable, but requires less maintenance than a stranded electric fence and, if something goes wrong with the electrical part, the fence can still act as a physical barrier. The barrier element means that a combination fence is mainly suitable where the same area needs protection each breeding season and the fence can be left in place (see the Coquet Island case study). Combination fences are more expensive than semi-permanent electric fence.
Disturbance-reduction fences
A symbolic single strand rope fencing is often used to discourage human visitors and their dogs from approaching too close to beach nesting birds such as little terns or least terns. This type of fence can be deployed or adjusted quickly, especially if thin fibreglass poles which can be easily pounded into the sand are used as fenceposts.
There will often be an electric or barrier fence a short distance within the rope fence, and the rope acts to keep people a safe distance from this. The most effective way of reducing human disturbance is usually a combination of rope and electric fencing combined with wardening to engage people and explain the need to protect the birds. Most people are also more likely to behave in what they believe is a reasonable manner if they think they are being observed.
A symbolic single strand rope fencing is often used to discourage human visitors and their dogs from approaching too close to beach nesting birds such as little terns or least terns. This type of fence can be deployed or adjusted quickly, especially if thin fibreglass poles which can be easily pounded into the sand are used as fenceposts.
There will often be an electric or barrier fence a short distance within the rope fence, and the rope acts to keep people a safe distance from this. The most effective way of reducing human disturbance is usually a combination of rope and electric fencing combined with wardening to engage people and explain the need to protect the birds. Most people are also more likely to behave in what they believe is a reasonable manner if they think they are being observed.
Signage is another way of informing and potentially influencing peoples behaviour. The RSPB has trialled using a sequence of signs to engage dog walkers, starting with a ‘welcome’ sign featuring an image of a happy dog to engage dog walkers, then next signs inform them of the need to keep dogs on a lead and why, and provide alternative routes. Finally the signs nearest the tern colony emphasise the risk to a dog of approaching the electric fence and feature a photo of a cute but vulnerable-looking puppy. This approach is still in development and has not yet been scientifically evaluated, however anecdotally it seemed to be well-received and effective.
Figure 11. A welcome sign in zone 1 of the RSPB trial of signage to influence dog walker behaviour around beach-nesting birds (RSPB).
Other mammal deterrents: sonic deterrents
On the Copeland Islands at the mouth of Belfast Loch, ultrasonic and light deterrents were deployed in combination in response to a particularly severe season of otter predation on Manx shearwaters Puffinus puffinus and were believed to have been effective in reducing predation (Leonard, 2011). However, this was not a scientific study and other studies have found no effect of using such devices, so the results must be considered inconclusive.
On the Copeland Islands at the mouth of Belfast Loch, ultrasonic and light deterrents were deployed in combination in response to a particularly severe season of otter predation on Manx shearwaters Puffinus puffinus and were believed to have been effective in reducing predation (Leonard, 2011). However, this was not a scientific study and other studies have found no effect of using such devices, so the results must be considered inconclusive.
Published Research
A study at Sands of Forvie, eastern Scotland (Forster, 1975) found that the number of Sandwich terns Thalasseus sandvicensis nesting in a colony increased from approximately 80 pairs in 1973 to approximately 450 pairs in 1974, following the erection of a 45cm high electric fence to separate the colony from the mainland. Previous low numbers were attributed to fox predation, and after the fence was erected there was only one case of a fox recorded breaching the fence and this animal did not approach the terns.
A study in 1978 on a beach in Massachusetts, USA (Minsky, 1980) found that the number of least tern nests in a colony decreased from 138 to 45 between counts on the 20th and 23rd June (fox tracks were found in the colony). An electric fence was erected around the colony on the 24th June and the number of nests increased to 85 with no new fox tracks found in the colony. No nests outside the fence survived. In total, 27 chicks fledged from the colony, nearly all from eggs laid after the erection of the fence.
A study in Massachusetts, USA (Rimmer & Deblinger, 1992) found that the proportion of least tern nests lost to predation was significantly lower in two colonies protected in 1990 - 1991 by 1.2 m high wire-mesh fencing, compared to three unprotected colonies over the same time period. Predation was also lower in the study colonies after fencing compared to nine colonies without fencing between 1987 and 1991.
A trial from 1991 - 1994 on alkaline flats in Oklahoma, USA (Koenen et al. 1996) found that the nesting success (i.e. at least one egg hatching) of least terns was significantly higher inside two electric fence enclosures than outside. The proportion of tern eggs predated (mainly by coyotes) was lower inside the fence. The fence was 86 cm high.
A study at Sands of Forvie, eastern Scotland (Forster, 1975) found that the number of Sandwich terns Thalasseus sandvicensis nesting in a colony increased from approximately 80 pairs in 1973 to approximately 450 pairs in 1974, following the erection of a 45cm high electric fence to separate the colony from the mainland. Previous low numbers were attributed to fox predation, and after the fence was erected there was only one case of a fox recorded breaching the fence and this animal did not approach the terns.
A study in 1978 on a beach in Massachusetts, USA (Minsky, 1980) found that the number of least tern nests in a colony decreased from 138 to 45 between counts on the 20th and 23rd June (fox tracks were found in the colony). An electric fence was erected around the colony on the 24th June and the number of nests increased to 85 with no new fox tracks found in the colony. No nests outside the fence survived. In total, 27 chicks fledged from the colony, nearly all from eggs laid after the erection of the fence.
A study in Massachusetts, USA (Rimmer & Deblinger, 1992) found that the proportion of least tern nests lost to predation was significantly lower in two colonies protected in 1990 - 1991 by 1.2 m high wire-mesh fencing, compared to three unprotected colonies over the same time period. Predation was also lower in the study colonies after fencing compared to nine colonies without fencing between 1987 and 1991.
A trial from 1991 - 1994 on alkaline flats in Oklahoma, USA (Koenen et al. 1996) found that the nesting success (i.e. at least one egg hatching) of least terns was significantly higher inside two electric fence enclosures than outside. The proportion of tern eggs predated (mainly by coyotes) was lower inside the fence. The fence was 86 cm high.
Figure 12. The line of the Hodbarrow fence (RSPB)
Case Studies
RSPB Hodbarrow: an aquatic fence on site without full-time wardening
At Hodbarrow (see habitat creation and restoration), a low maintenance anti-predator fence was needed to protect tern colonies from fox predation. The site is not wardened full-time and has a moderate threat of vandalism/unauthorised access which precluded the use of high-maintenance electric fencing requiring the using batteries or solar panels etc. However, as a former iron mine slag heap, aesthetic considerations were less restrictive than might be the case at more natural sites.
The fence was designed, manufactured and installed by a local fencing contractor and was adapted from the standard security fencing which the company install around schools, businesses etc. The panels are semi-rigid coated steel wire 1.8 m high by 2.4 m long attached to 60 mm square hollow section steel posts. The panels are bolted onto the uprights via a clamp into threaded aluminium inserts (see below). Both panels and uprights are fully galvanised and polycoated.
The uprights were modified by welding a 600 x 600 mm, 3 mm thick steel plate to the base. When placed in the water, the steel plate either settled into the sediment or occasionally needed to be weighed down with a concrete block. The installation of the 400 m long fence was undertaken by a team of four and took approximately 3 days.
The wire panels can be bolted onto the uprights at any height, allowing for variation in water heights. The panels were set approximately 0.6 m below the water surface (almost to the lagoon bottom) and 1.2 m above the water surface. One section was left unbolted and hung on pegs welded to the uprights to allow the panel to be removed for access. This can be chained and padlocked to prevent unauthorised access.
RSPB Hodbarrow: an aquatic fence on site without full-time wardening
At Hodbarrow (see habitat creation and restoration), a low maintenance anti-predator fence was needed to protect tern colonies from fox predation. The site is not wardened full-time and has a moderate threat of vandalism/unauthorised access which precluded the use of high-maintenance electric fencing requiring the using batteries or solar panels etc. However, as a former iron mine slag heap, aesthetic considerations were less restrictive than might be the case at more natural sites.
The fence was designed, manufactured and installed by a local fencing contractor and was adapted from the standard security fencing which the company install around schools, businesses etc. The panels are semi-rigid coated steel wire 1.8 m high by 2.4 m long attached to 60 mm square hollow section steel posts. The panels are bolted onto the uprights via a clamp into threaded aluminium inserts (see below). Both panels and uprights are fully galvanised and polycoated.
The uprights were modified by welding a 600 x 600 mm, 3 mm thick steel plate to the base. When placed in the water, the steel plate either settled into the sediment or occasionally needed to be weighed down with a concrete block. The installation of the 400 m long fence was undertaken by a team of four and took approximately 3 days.
The wire panels can be bolted onto the uprights at any height, allowing for variation in water heights. The panels were set approximately 0.6 m below the water surface (almost to the lagoon bottom) and 1.2 m above the water surface. One section was left unbolted and hung on pegs welded to the uprights to allow the panel to be removed for access. This can be chained and padlocked to prevent unauthorised access.
The fence was installed in a zig-zag pattern to increase stability against wind and wave action. It is possible that the internal corners on the fence might enable a determined fox to climb the fence. However, it would be possible to retrofit a ‘floppy top’ of overhanging wire mesh to the top of the fence, at least at the internal corners, if attempts to climb the panels by foxes are detected.
The effectiveness of the fence has been monitored by use of a thermal imager, trail cameras and scat transects throughout the breeding season. There were several night-time sightings of a fox attempting (unsuccessfully) to get around the fence by swimming up and down it before returning to the mainland.
The effectiveness of the fence has been monitored by use of a thermal imager, trail cameras and scat transects throughout the breeding season. There were several night-time sightings of a fox attempting (unsuccessfully) to get around the fence by swimming up and down it before returning to the mainland.
Coquet Island, Northumberland, England: otter fencing the UKs only roseate tern colony
Coquet Island supports a large seabird colony with a breeding assemblage of over 10 species and over 40,000 pairs, including common, Arctic terns Sterna paradisaea and Sandwich terns as well as the UK’s only breeding population of roseate terns (130 pairs in 2020). The island is managed by the RSPB as a nature reserve, with a particular focus on the breeding roseate terns. There is no public access.
In January 2019, otter scat and tracks were discovered on the island and shortly afterwards, images of the animal were captured by trail positioned at points around the island where spraints and feeding remains had been found.
Coquet Island supports a large seabird colony with a breeding assemblage of over 10 species and over 40,000 pairs, including common, Arctic terns Sterna paradisaea and Sandwich terns as well as the UK’s only breeding population of roseate terns (130 pairs in 2020). The island is managed by the RSPB as a nature reserve, with a particular focus on the breeding roseate terns. There is no public access.
In January 2019, otter scat and tracks were discovered on the island and shortly afterwards, images of the animal were captured by trail positioned at points around the island where spraints and feeding remains had been found.
Figure 15. Trailcam picture of the Coquet otter (RSPB)
Since otters are a protected species in the UK the options for dealing with the incursions were limited; fencing the entire island was clearly not an option so a decision was made to quickly install temporary electric fences around the two roseate tern nesting terraces before the terns returned to the island in April. The location required fixing metal fencing stakes into sandstone bedrock, on low cliff edges and on very uneven ground into loose, friable sandy soils.
Figure 16. The temporary otter fence installed on Coquet Island between puffin burrows and across bare sandy soil (Paul Morrison; RSPB)
Although the otter did not breach the temporary fence , there were a number of ongoing issues with it including the loose sandy substrate which made tensioning wires difficult, and also nest-building black-headed gulls Chroicocephalus ridibundus regularly dropping seaweed onto the wires, and burrowing puffins Fratercula arctica covering the wires with sand heaps both of which caused shorting of the fence.
Dealing with these fence maintenance issues also caused increased disturbance to the tern colony. Night patrols were made to detect and prevent otters from attacking the colony. Fortunately, despite the challenges there were no recorded otter incursions into the roseate tern terraces and there was good productivity from the 122 pairs of roseate terns.
The otter appeared to concentrate on hunting puffins and it was estimated from camera evidence that the otter was taking two puffins a night. As the mainland is less than a mile away at low tide, it was unclear whether the otter was resident on the island or swimming over each day. After the birds started to leave the island, sightings became infrequent and the animal wasn’t seen after the end of July. Once the season had ended the search for a more permanent solution began, since otter numbers are increasing in Northumberland it was considered likely that otters would return. A more robust permanent combination electric fence was installed around the roseate terraces early in 2020.
The new fence was supplied by a local fencing contractor and installed by a team of two over four days. The fencing surrounds the two roseate tern terraces, with perimeters of approximately 68 m and 117 m. It uses Tornado C5 Torus Otter fencing wire (code 19/180/5) fixed to 1.8 m round creosote treated timber posts topped by metal brackets. The posts are spaced no more than every 2 m with straining posts at strainers to corners/ends of fencing and supporting gateways. The posts and wire are buried to depth of 800 mm where soil/ ground conditions allow and on rock the posts are cut down to 1 m and pinned down tight with fixings drilled into the rock. The height of the fence is 1 m plus the height of wire netting ‘turned outwards’ at top of fence using galvanised angle brackets (see picture below). There are otter-proof self-closing gateways into each terrace. There are two lines of electric wire around each of the two enclosed areas (even at the gates). The lower wire is about 60 cm from the ground and the upper one a further 20 cm above that. Both wires are shielded from being shorted out, particularly by seaweed dropped by black-headed gulls, by the overhanging mesh top (Figures 18 and 19). The electricity is powered by a Gallagher S16 Solar fence energiser. The double strand is a continuous live loop to which the positive lead from the energiser is attached at a convenient place where the energiser can be left facing south, without risk of shadows from the fence or other structures.
Once the seabirds returned to Coquet in spring 2020, there were a few incidents of puffins getting tangled in the fence. To prevent this a 0.5 m high fine mesh (10 mm square) plastic netting was fixed along the entire length of the fence to stop puffins and eider ducklings from getting trapped. The fence was also modified by cutting “Puffin Escapes” at four points along the fence so if they get stuck they follow the fence line until they reach an “escape” and slip through to freedom. When no otter is present the gates are also left open, so trapped birds have an escape route. Mesh gates are made up ready to shut them off should an otter be detected on the island. Finally, on the inside of the north terrace enclosure, the Coquet staff constructed a high mound of soil, the height of the fence so trapped puffins can climb up and fly off the top.
Figure 20. The new Coquet otter fence seen from below
Beacon Ponds, Easington Lagoon: temporary fencing a little tern colony
On the coast of East Yorkshire, UK, north of Spurn peninsula, at the mouth of the River Humber the site comprises a variety of coastal habitats including saltmarsh, shingle, sand dune, swamp and most significantly, saline lagoons. Beacon Ponds is a Site of Special Scientific Interest and part of a larger Special Protection Area; while the Spurn Peninsula is a National Nature Reserve. Little terns nest on the sand dune and shingle storm beach seaward of the northern lagoon. The little tern colony is managed by the Beacon Ponds Little Tern Project, a joint venture of Spurn Bird Observatory, the South Holderness Countryside Society, the RSPB, the Yorkshire Wildlife Trust and the Environment Agency. The project supports wardening of the little tern colony throughout the breeding season.
The nature of the site means that a permanent barrier or combination fence would be impractical, and there is no need to prevent public access to the little tern nesting area when the birds are not present. Instead, as is in the case with many little tern breeding sites in the UK, the colony is protected by a combination of a symbolic perimeter fence to remind people, particularly dog walkers, not to get too close to the colony and by a temporary electric anti-predator fence, both of which are installed for the breeding season. Because the same area is fenced each season the fenceposts for the perimeter fence and the corner/strainer posts for the anti-predator fence, which are wood, are left in place, although the nature of the ground means that they may require resetting every year. The fence consists of 12 strands and a tripwire is installed about 60 cm outside the fence and about 30 cm off the ground. This is separately electrified so that a fox trying to jump over or dig under both parts of the fence is unlikely to avoid being shocked.
On the coast of East Yorkshire, UK, north of Spurn peninsula, at the mouth of the River Humber the site comprises a variety of coastal habitats including saltmarsh, shingle, sand dune, swamp and most significantly, saline lagoons. Beacon Ponds is a Site of Special Scientific Interest and part of a larger Special Protection Area; while the Spurn Peninsula is a National Nature Reserve. Little terns nest on the sand dune and shingle storm beach seaward of the northern lagoon. The little tern colony is managed by the Beacon Ponds Little Tern Project, a joint venture of Spurn Bird Observatory, the South Holderness Countryside Society, the RSPB, the Yorkshire Wildlife Trust and the Environment Agency. The project supports wardening of the little tern colony throughout the breeding season.
The nature of the site means that a permanent barrier or combination fence would be impractical, and there is no need to prevent public access to the little tern nesting area when the birds are not present. Instead, as is in the case with many little tern breeding sites in the UK, the colony is protected by a combination of a symbolic perimeter fence to remind people, particularly dog walkers, not to get too close to the colony and by a temporary electric anti-predator fence, both of which are installed for the breeding season. Because the same area is fenced each season the fenceposts for the perimeter fence and the corner/strainer posts for the anti-predator fence, which are wood, are left in place, although the nature of the ground means that they may require resetting every year. The fence consists of 12 strands and a tripwire is installed about 60 cm outside the fence and about 30 cm off the ground. This is separately electrified so that a fox trying to jump over or dig under both parts of the fence is unlikely to avoid being shocked.
Figure 22. An extract from the detailed instructions on fence installation provided to the seasonal little tern wardens at Beacon Ponds (Bennet, 2013).
Relevant sections of Conservation EvidenceAction: Protect bird nests using electric fencing
Action: Physically protect nests from predators using non-electric fencing
Action: Physically protect nests from predators using non-electric fencing
Links
Rappa: electric fence supplier - useful product information
Otterstop: otter fence specialist with useful product information
Rappa: electric fence supplier - useful product information
Otterstop: otter fence specialist with useful product information
References
Bennet, H. (2013) A Guide for Little Tern Wardens. Unpublished manual prepared for the Beacon Ponds Little Tern Project
Forster J.A. (1975) Electric fencing to protect sandwich terns against foxes. Biological Conservation, 7
Koenen M.T., Utych R.B. & Leslie D.M. Jr. (1996) Methods used to improve least tern and snowy plover nesting success on alkaline flats. Journal of Field Ornithology, 67, 281-291
Leonard K.S. Copeland Islands Special Protection Area - Impact of Otter predation on selection features. 2011. Report to the Northern Ireland Environment Agency.
McKillop, I.G., Pepper, H.W., and Butt, R., 1999. Electric fencing reference book. Produced for DEFRA.
Minsky D. (1980) Preventing fox predation at a least tern colony with an electric fence. Journal of Field Ornithology, 51, 180-181
Rimmer D.W. & Deblinger R.D. (1992) Use of fencing to limit terrestrial predator movements into least tern colonies. Colonial Waterbirds, 15, 226-229
Smith, R. K., Pullin, A. S., Stewart, G. B., & Sutherland, W. J. (2010). Effectiveness of predator removal for enhancing bird populations. Conservation Biology, 24, 820–829
White G. and Hirons G. (2019) The Predator Exclusion Fence Manual: Guidance on the use of predator exclusion fences to reduce mammalian predation on ground-nesting birds on RSPB reserves. Version 3. RSPB Guidance Document
AcknowledgementsThanks to: Paul Morrison (RSPB Coquet Island), Dave Blackledge (RSPB Hodbarrow), Richard Archer (RSPB Chesil Little Tern Project), David Steel (Scottish Natural Heritage Isle of May), Richard Boon (Beacon Ponds Little Tern Project).
Bennet, H. (2013) A Guide for Little Tern Wardens. Unpublished manual prepared for the Beacon Ponds Little Tern Project
Forster J.A. (1975) Electric fencing to protect sandwich terns against foxes. Biological Conservation, 7
Koenen M.T., Utych R.B. & Leslie D.M. Jr. (1996) Methods used to improve least tern and snowy plover nesting success on alkaline flats. Journal of Field Ornithology, 67, 281-291
Leonard K.S. Copeland Islands Special Protection Area - Impact of Otter predation on selection features. 2011. Report to the Northern Ireland Environment Agency.
McKillop, I.G., Pepper, H.W., and Butt, R., 1999. Electric fencing reference book. Produced for DEFRA.
Minsky D. (1980) Preventing fox predation at a least tern colony with an electric fence. Journal of Field Ornithology, 51, 180-181
Rimmer D.W. & Deblinger R.D. (1992) Use of fencing to limit terrestrial predator movements into least tern colonies. Colonial Waterbirds, 15, 226-229
Smith, R. K., Pullin, A. S., Stewart, G. B., & Sutherland, W. J. (2010). Effectiveness of predator removal for enhancing bird populations. Conservation Biology, 24, 820–829
White G. and Hirons G. (2019) The Predator Exclusion Fence Manual: Guidance on the use of predator exclusion fences to reduce mammalian predation on ground-nesting birds on RSPB reserves. Version 3. RSPB Guidance Document
AcknowledgementsThanks to: Paul Morrison (RSPB Coquet Island), Dave Blackledge (RSPB Hodbarrow), Richard Archer (RSPB Chesil Little Tern Project), David Steel (Scottish Natural Heritage Isle of May), Richard Boon (Beacon Ponds Little Tern Project).