Australian birds could benefit from predator exclusion fencing

The use of predator exclusion fencing is widely recognized as one of the most effective mechanisms for protecting threatened fauna from introduced or overabundant predators. It is now used throughout the world to protect avian fauna. In Australia, where predator exclusion fencing is used extensively to protect threatened mammals, such fences for threatened avifauna are surprisingly sparse. In this perspective we demonstrate that (a) the use of predator exclusion fences for avifauna is popular in other countries with similar conservation problems, (b) a large number of Australian avian species could benefit from dedicated predator exclusion fences, and (c) despite legitimate concerns, if well designed, predator exclusion fences can pose little risk to threatened birds or are outweighed by the potential benefits. We believe wider use of predator exclusion fencing to protect Australian threatened birds could be highly beneficial and should be more regularly considered as a management action by conservation practitioners.


| INTRODUCTION
Introduced cats (Felis catus) and foxes (Vulpes vulpes) are leading causes of extinction and decline in Australian wildlife (Kearney et al., 2019). Their greatest impact has been felt by mammals (Radford, Woinarski, Legge, Baseler, & Bentley, 2018), however, they have also had a massive impact on birds . Seventy-one out of Australia's 117 listed bird species (61%) are recognized to be predated by either cats . Cats especially pose a significant threat to Australia's birds, with evidence suggesting that they kill some 400 million birds every year , many of which are threatened taxa (Woinarski, Woolley, et al., 2017). For many Australian bird species, ongoing predator control is essential to prevent further declines and avoid extinction.
Australia has trialed numerous methods of predator control to combat the impact of feral predators on native birds. Predator control methods like poison baiting, trapping and shooting have had mixed success (Marlow et al., 2015), but fencing to exclude predators has high efficacy (Clayton, Pavey, Vernes, & Tighe, 2014) and is well received by the public (van Eeden, Newsome, Crowther, Dickman, & Bruskotter, 2019). Fencing is an attractive management option because it offers a consistent and highly effective control mechanism, albeit at a high initial investment (Bode, Brennan, Morris, Burrows, & Hague, 2012).
Fencing describes a structure used to restrict the movement of an agent, which may not necessarily be physical (e.g., the use of sound). Fencing can act as both an asset and a hinderance in conservation, depending on its design and intent (Hayward & Kerley, 2009). Conservation fencing-a relatively recent term, describes a fence specifically created for aiding the conservation of biodiversity. Many forms of fencing can have benefits to biodiversity, for example wildlife barrier fences near highways prevent roadkill (Clevenger, Chruszcz, & Gunson, 2001), and many fences serve to reduce the impact of wildlife on agriculture (Dickman, 2010), which consequently can abate human-wildlife conflict. Hence, the classification of true conservation fences constructed for the benefit of biodiversity as its primary objective can be ambiguous.
Here, we argue that predator exclusion fencing-a specific form of conservation fencing created to protect native fauna from introduced or overabundant predators (see Figure 1) is an underutilized, high-impact management action for the conservation of birds in Australia. Australia is a prolific builder of often massive (with several examples in excess of 8,000 ha) predator exclusion fences to protect native mammals Ringma et al., 2019), but has not followed suit for birds. Despite their upfront costs, the long-term dividends of fences often make them cost-effective control methods over ecological time-scales. The conservation sector contains a diversity of organizations, values and priorities. Many of these organizations might pursue alternative conservation actions, which better reflect their values and resource limitations. Nevertheless, a wider adoption of fencing as a predator control method could make a meaningful and immediate contribution to the conservation of at-risk birds. There is a strong precedent for the use of fences, compelling reasons to believe they would be effective for birds, and many species that would benefit.

| AUSTRALIA SHOULD USE MORE PREDATOR EXCLUSION FENCING FOR BIRDS
Around the globe, predator exclusion fencing has been used extensively to protect birds from introduced predators. To date, over 50 fences have been erected for the protection of nesting seabirds (Cooper, 2013). Predator exclusion fences in New Zealand alone protecting more than 30 bird species behind 28 fences (Burns, Innes, & Day, 2012), 15 fences >25 ha; Innes et al., 2019), with reports of widespread recovery of forest birds and restored ecosystem function as a result of predator exclusion fencing (Bombaci, Pejchar, & Innes, 2018;Miskelly, 2018). In Europe (Mayer & Ryan, 1991) and North America (Malpas et al., 2013), game birds and waders are regularly protected using both long term fencing such as peninsula fences, and short-term fencing to opportunistically protect individual nests. These have achieved remarkable successes Young et al., 2013), and when appropriately designed, result in negligible adverse effects such as strike (D. Shanahan, personal communication). Even for species particularly prone to strike, such as seabirds, the risk can be reduced through the use of high visibility tape and careful fence design (e.g., matching fence boundaries to topographic features (Swift, 2004;L. Young, personal communication).
In Australia, only a single fence has been constructed whose primary purpose is to protect a bird from introduced predators; it is at Little Desert for the groundnesting malleefowl (Leipoa ocellate; Benshemesh, 2007). This is not to say Australian birds are not already benefiting from fences: Fences constructed for mammals provide latent protection for dozens of species, and bush stonecurlews (Burhinus magnirostris) have been translocated into some of these fence sites (Shorthouse et al., 2012). Fences to prevent stock and pigs from damaging wetlands (Doupé, Mitchell, Knott, Davis, & Lymbery, 2010;Kutt, 2017) also improve habitat quality and ecosystem level stresses on bird populations. However, targeted control of introduced predators through fencing with the primary objective of conserving bird populations in Australia is remarkably scarce, despite the potential benefits. negative effects onbiodiversity. Conservation fencing is a subset of all fencing types whichrelates specifically to fences constructed with the primary purpose ofbenefiting biodiversity. In this article, we refer mainly to predator exclusionfencinga form of conservation fencing whose primary purpose is to create abarrier between invasive or overabundant predators and sensitive prey species It is not certain why predator exclusion fencing has not been more widely used to protect Australian birds. Perhaps construction costs have been thought prohibitively expensive (Norbury, Hutcheon, Reardon, & Daigneault, 2014;Scofield, Cullen, & Wang, 2011), or the perceived threat of bird strike has resulted in a tendency toward inaction or alternate intervention (e.g., due to zero-risk bias, Schneider, Streicher, Lermer, Sachs, & Frey, 2017). Yet similar concerns raised for mammals in Australia and for bird taxa have been weighed in favor of predator-exclusion fencing in other countries. Moreover, conservation fencing is commonly used in Australia to protect bird habitat from herbivory (e.g., ungulate exclusion fencing is now a common management technique in tropical wetlands, Doupé et al., 2010) and fences are frequently constructed as a boundary demarcation of protected areas, typically with little regard to bird strike risk, which arguably pose a high strike risk due to their design and visibility.
We suggest that predator exclusion fences could be used to protect a wide range of Australian bird species, particularly those vulnerable to cat and fox predation, especially either ground nesting or ground foraging taxa. For example, 109 of 162 (67%) ground nesting birds which breed in Australia have been recorded to be predated by cats (Garnett et al., 2015;Woinarski, Woolley, et al., 2017). Terrestrial species such as Golden-shouldered parrots (Psephotus chrysopterygius), night parrots (Pezoporus occidentalis), ground parrots (Pezoporus flaviventris and Pezoporus wallicus), and plains wanderers (Pedionomus torquatus) all face extremely high predation from introduced predators. Many of these species currently persist in the presence of strand wire stock fencing, which is arguably less visible to birds than a mesh construction predator exclusion fence. It is our conjecture that strike risks from thoughtfully constructed fencing would likely be outweighed by the benefit of removing predators-as seen with countless other examples of predator exclusion fences made for the benefit of birds throughout the world. When in doubt, structured decision making frameworks can assist managers at deciding when the risk of bird strike outweighs the benefits predator control (Canessa, Ewen, West, McCarthy, & Walshe, 2016).
Wader and waterfowl species in Europe and North America are frequently protected by fences (Malpas et al., 2013;Mayer & Ryan, 1991), these birds' nest and forage in predictable areas, meaning that both targeted fencing of nest sites, or landscape-scale fences could improve survival during vulnerable life-stages. For example, 32 of 45 (71%) Charadriiformes and nine out of 12 (75%) Procellariformes are recognized to be predated by cats (Garnett et al., 2015;Woinarski, Woolley, et al., 2017). Fences could be used to assist seabirds and waders such as fairy tern (Stena neris), little penguins (Eudyptula minor) and hooded plovers (Thinornis cucullatus) which breed on the Australian mainland, and are in decline due to predation. High tide wader roosts and other critical habitats, many of which are often managed with poison baiting could similarly receive a higher level of protection through fencing. For this application, small scale fencing provides unparalleled protection while being the most cost-efficient predator control option (Norbury et al., 2014). Similarly, the plains wanderer is recognized to be threatened by foxes, yet fencing of known habitat to exclude foxes is not even identified as a potential action in the recovery plan (Department of Environment, 2015). Instead broadscale baiting across complex tenure is proposed, which is likely to carry higher costs for lower gains (particularly given recolonization rates of foxes in agricultural landscapes; Gentle, Saunders, & Dickman, 2007), and the risk of perverse outcomes from trophic cascade responses (i.e., potential increase in cat abundance; Marlow et al., 2015).
The benefits of predator exclusion fencing to conserve threatened birds outweigh the costs. With a growing body of evidence in support of fencing as a conservation action and worsening conservation crisis, the costs of continuing to ignore the benefits of fencing for birds are too great to continue being overlooked.