Nesting Refuge Structures Are Ineffective at Protecting Bell's Turtle (Myuchelys bellii) Nests from Red Fox (Vulpes vulpes) Depredation

In biodiversity terms, Australia is as much a gigantic island as it is a small continent (Simpson 1961). Prolonged isolation from the rest of the Earth's landmasses has resulted in a wide variety of unique Australian lifeforms and communities (Simpson 1961; Lomolino and Channell 1995). Like many islands, however, Australia's species and ecosystems are vulnerable to invasion (Mooney et al. 2005). When European explorers and settlers arrived in Australia, they brought rats (Rattus sp.), mice (Mus musculus), cats (Felis catus), and rabbits (Oryctolagus cuniculus) among other animal and plant species, which have all taken a heavy toll on native Australian wildlife communities through depredation, competition, or displacement (Bell et al. 2003; Abbott 2011). The invasive mammalian predators have had a particularly devastating impact on numerous reptile populations through direct predation (Woinarski et al. 2018a, 2018b). In the context of Australian freshwater turtle conservation, predation of nests and nesting female predation by the red fox (Vulpes vulpes) is of paramount concern (Thompson 1983; Van Dyke et al. 2019). Freshwater turtles are largely protected in the adult life stage; however, they are vulnerable to these terrestrial predators when returning to land to nest (Spencer 2002).

Red foxes are generalist meso-predators with a naturally Holarctic distribution, and were introduced to Australia in the 1860s or 1870s, probably for sport (Saunders et al. 2010; Abbott 2011). Since then, the species has spread across much of the continent, where they generally outcompete native meso-predators (Glen and Dickman 2008) and have caused declines in several native prey species (Mahon 2009; Abbott 2011). The impact of red foxes on Australian freshwater turtles has been highlighted by Van Dyke et al. (2019), who argue that there is a pressing need to devise management strategies for freshwater turtles that address fox depredation of turtle nests. The endangered Bell's turtle (Myuchelys bellii) is a frequent victim of nest raiding by foxes (New South Wales Office of Environment and Heritage [NSW OEH] 2014). Bell's turtles are a species of freshwater turtle found in the west-flowing streams of the New England Tablelands in northern New South Wales and southern Queensland (Fielder et al. 2015a). They are a highly aquatic species, and although they will leave the water to bask on rocks or logs, nesting female Bell's turtles generally stay close to the water's edge, nesting on the slopes of the riverbank (NSW OEH 2014; Fielder et al. 2015a; Cann and Sadlier 2017). Fox depredation of Emydura macquarii nests have been reported to exceed 96% in some years (Thompson 1983; Spencer 2002), and it is suspected that fox predation of M. bellii nests may also be high where fox control is minimal or absent (NSW OEH 2014; Fielder et al. 2015b). Bell's turtle populations are measurably ageing with little apparent recruitment (Chessman 2015), leading to concerns of an incipient population crash in the future if measures are not taken to increase immediate recruitment.

Worldwide, various methods have been trialed in attempts to protect turtle nests and other vulnerable native species from fox depredation: poison baits, shooting and trapping, and nest caging are a few examples (Fagerstone et al. 2004; Gentle et al. 2007; Riley and Litzgus 2013). Although these methods show short-term successes (e.g., Spencer 2000), in the long term they are generally not effective or are extremely labor-intensive to maintain indefinitely (Harding et al. 2001; Gentle et al. 2007; Spencer et al. 2016, 2017). Additionally, some landholders and community members may be reluctant to allow lethal fox control methods on their properties, for fear of pets and livestock being harmed. As such, it is important to consider additional protection methods, which can be used in lieu of, or in concert with, these existing strategies.

While nest caging is an effective method of nest protection (Riley and Litzgus 2013), protecting a small area of nesting beach and enticing female turtles to nest within is an alternative that has seen success in previous studies (e.g., Quinn et al. 2015), and if successful could be employed on a broad scale to increase recruitment. In this study, a nesting refuge structure design was tested in the summers of 2019–2020 and 2020–2021 at sites within the Bell's turtles' range. These structures were intended to allow female turtles to nest within while excluding foxes; the females could thus nest unharmed, and the embryos could develop without the nest being raided. Intensive searches with researchers and dogs for individual nests would not be required, only construction of the refuge structure prior to commencement of the nesting season. This study had sought to test two questions: will female Bell's turtles use the structures as designed, and will the structures successfully exclude foxes? It was hypothesized that these fox-exclusion structures could increase recruitment of Bell's turtles because the lack of positive stimulus for foxes of obtaining eggs or adult turtles as prey items would not reinforce foraging activity on nesting beaches. It was predicted that these structures would allow female Bell's turtles to enter and nest unharassed, while excluding red foxes and other potential predators (e.g., corvids, pigs). If successful, these nest refuges could provide a long-term, cost-effective method of protecting nests from fox predation, creating a tool for Bell's turtle conservation efforts.

METHODS

Nesting Refuge Structures. — Six nesting refuge structures were constructed and placed in selected sites across the New England Tablelands where Bell's turtle nesting activity was known or anticipated (Fig. 1); nesting had been previously recorded at 5 of 6 sites, and mature females had been captured at all 6 sites (M. Dillon, pers. comm., March 2020). These structures were based on designs successfully employed by Quinn et al. (2015) for diamondback terrapins (Malaclemys terrapin) in the United States. Bell's turtles live in winding upland streams with loam soil and granite outcrops, and females nest close to the water's edge on the riverbank (NSW OEH 2014; Fielder et al. 2015b; Cann and Sadlier 2017). The nest refuge structures were deployed individually at different sites, with the placement of the structure and additional equipment modified as best as possible to fit the topography of each location and the needs of the species.

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The nest refuge structures consisted of wooden frames (2.4 m long × 1.8 m wide × 0.6 m tall), with chicken wire covering all sides except the entrance and bottom (Fig. 1). The entrance was 15 cm tall with an electric fence wire (100 × 12-mm polytape) placed along the top of the entrance (Fig. 1); this was intended to allow turtles into the structure while excluding foxes and other predators. The wire was energized with a Thunderbird S18B solar fence charger (Thunderbird Ag, Mudgee NSW, Australia) set to full charge. Drift fencing (4–6 m in length depending on the position of the structure relative to the water's edge) was placed at either side of the entrance and ran outward at an angle (dependent on the shape of the beach) to funnel nest-searching females into the entrance. This drift fencing was composed of 30-cm-tall rigid plastic garden edging in 2019–2020 and was replaced with a 50-cm-tall flexible rubber mesh in 2020–2021 (Fig. 1).

The structures were built and deployed in November 2019 to January 2020 (during the austral summer). The structures were redeployed from November 2020 to January 2021 with some modifications to the original design. As mentioned, the drift fencing was replaced with a different material; additionally, the wooden beam at the bottom of the structure entrance was removed and replaced with metal mesh buried 5 cm deep to prevent foxes from burrowing under the electric wire, and half of the structure's top was replaced with a removable frame to allow researchers easier access to the interior. These changes aimed to reduce any aversion to the structures by increasing the size of the entrance.

A rotary hoe was used to churn patches of soil close to the river's edge, over which the structure was set. A mattock was used to dig a trench in the outline of the structure down to the hard earth beneath the tilled soil, to reduce the chances of foxes digging under the structure and bypassing the entrance. The edges of the structure were lowered into this trench with the entrance facing the river, and the edges buried; the soil inside the structure was raked to evenly distribute disturbed soil within the structure. In 2019–2020, the soil inside of the structure was also doused with ∼ 10 l of water taken from the river with a garden watering can; the interior of the structure was doused with each subsequent site visit, to simulate rainfall moistening soil. Disturbed soils have been noted as potential attractors to nesting Bell's turtles (G. Hughes, pers. obs.; P. Spark, pers. comm., August 2018), and rainfall is commonly associated with the start of nesting season in other turtle species (e.g., Mortimer and Carr 1987; Burke et al. 1994; Czaja et al. 2018). Conditions changed between field seasons from severe drought in the 2019–2020 field season (Filkov et al. 2020), through to much higher rainfall in the 2020–2021 season; water dousing was thus discontinued in the second season.

The structure was surrounded with electric fence to prevent livestock from approaching and potentially damaging the structure or being injured, and also to further make digging under the sides of the structure difficult for foxes (Fig. 1a). Two lines were run around the exterior of the structure on posts, one at ∼ 10 cm above the surface and one ∼ 50 cm above the surface; these were connected to the fence charger. This exterior fencing was removed in 2020–2021 out of concern that it discouraged female turtles from approaching the structure (Fig. 1b).

Each refuge structure had two camera traps monitoring the structures, either LTL Acorn camera traps (Little Acorn Australia, VIC, Australia) or Bushnell Trophycam camera traps (Bushnell Corporation, KS). Camera traps were always the same model at each site. In 2019–2020, one camera was placed at one side of the entrance, ∼ 30 cm from the structure and 10 cm above the ground, facing across the entrance. The second camera was placed beside the structure, facing outward to capture animals moving near the structure. In 2020–2021, both cameras were placed 2 m from the entrance on either side, ∼ 10 cm above the ground, facing inward at a 45° angle. These camera traps were intended to detect any turtles entering the structure, or any foxes or other predators interacting with the structure entrance. Sites were visited once per fortnight to exchange memory cards and replace batteries as necessary.

Study Sites. — Three sites were located in the Macdonald River catchment near Bendemeer, NSW, and three sites were located in the Gwydir River catchment near Bundarra, NSW (Fig. 2). For landholder anonymity, the property names and exact locations are not published so the sites are hereafter named Macdonald 1, 2, and 3 and Gwydir 1, 2, and 3. Some landholders engaged in active fox control (i.e., shooting or poison baiting) as outlined below, which potentially limited exposure of structures to foxes in this study.

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Macdonald 1 was located in an area of restored parkland. Invasive trees and weeds had been removed in the previous year, and the structure was deployed on 22 November 2020, on a slightly inclined patch of open, loamy soil, with the entrance 2.5 m from the water's edge and < 1 m above the waterline. There was shade to the south and northwest from remaining native trees (primarily Casuarinaceae), but the patch received full sun from directly overhead, and to the east and north. Flooding in January 2020 forced the structure to be relocated to a different location within the park for the 2020–2021 nesting season. The structure was modified and redeployed at a nearby site (∼ 100 m downstream) on 30 October 2020 on a higher bank (∼ 1.5 m above the waterline). Surrounding conditions were similar, with primarily Casuarinaceae forest to the south and open ground to the east, north, and west, ensuring sunlight coverage of the interior of the structure. In late December 2020 the area was again flooded, and the structure was irreparably damaged. Local landholders did not engage in fox control of any kind during these field seasons.

Macdonald 2 and Macdonald 3 were located on a private cattle and sheep farm on the Macdonald River. The two sites were ∼ 1 km apart and separated by a roadway and several fence lines; both structures were deployed on 22 November 2019. Macdonald 2 was located on a flat, sandy beach < 1 m above the waterline, ∼ 2 m from the edge of the water. There were no nearby trees or other sources of shade. Macdonald 3 was located on a high sand- and-gravel riverbank, with some shade from a large willow tree to the north but otherwise no obstacles to sun exposure. The same flooding event that threatened Macdonald 1 in January 2020 completely inundated and irreparably damaged Macdonald 2, and Macdonald 2 was not used in 2020–2021. Macdonald 3 was modified and redeployed 11 November 2020 and used for the 2020–2021 field season. Landholders engaged in fox control with 1080 (sodium fluoroacetate) baits on their property during these field seasons.

Gwydir 1 was located on a cattle and horse farm with intact riparian forest along the Gwydir River. The structure was placed on 18 November 2019 in a flat area of loamy soil, ∼ 2 m from the water's edge and 1 m above the water line. The site was heavily shaded by trees, but was open to the north and west, so was considered to receive enough sun exposure for the purposes of this experiment, and no more-suitable sites were available on the property. In January 2020 a flood event irreparably damaged the structure, and it was not used in the 2020–2021 field season. Landholders engaged in limited fox control (shooting only) on their property during these field seasons, reporting 4 foxes removed per year.

Gwydir 2 was located on a large (∼ 12,000 ha) cattle station on a tributary of the Gwydir River, which will remain confidential to protect the landholder's privacy. The structure was placed on 18 November 2020, in a flat area of loamy soil 2 m from the water's edge and 1 m above the water line. The site was shaded by a single willow tree to the south but had good sun exposure from all other angles. On 10 January 2020, a flood event slightly damaged the structure. The structure was repaired and modified and was redeployed on 3 November 2020 for the 2020–2021 field season; it was moved ∼ 80 m from the first site, with sandy soil and less shade; the new site was 2 m from the water and < 1 m above the waterline. Landholders engaged in intensive fox control (shooting only) on their property during these field seasons, reporting > 300 foxes killed over these 2 yrs across the whole property.

Gwydir 3 was located on a private cattle and sheep farm along a patch of open riparian woodland. The structure was placed on 27 November 2019, on a high bank (2 m) above the river with some shade from a large eucalypt to the north east but otherwise in a location well-exposed to sunlight. The structure was modified and reset on 3 November 2020 for the 2020–2021 field season but was irreparably damaged by flooding in January 2021. Landholders engaged in limited fox control (1080 baiting) on their property during the 2019–2020 field season but did not engage in fox control during 2020–2021.

Analysis. — Images from the camera traps were examined for Bell's turtle activity in or near the structures. Particular focus was placed on observing the turtles entering or exiting the structure, or attempting to nest (i.e., manipulating soil with hind legs). Additionally, signs of digging activity in or near the structures were visually searched for during site visits. Any raided turtle nests near the structures were also noted, in particular the distance between these nests and the structure's entrance, and distance to the water's edge.

Images taken by the camera traps were examined for fox activity in or near the structures. Attention was focused on observing the foxes investigating the structure entrances, and any interactions with the wires. Activity of other local egg-laying reptile species, such as eastern water dragons (Intellagama lesueurii) and eastern longneck turtles (Chelodina longicollis) were also noted, as potential indicators of nesting habitat viability. During site visits, any tracks, scat, or apparent attempts to dig under the structure from the sides were noted if found. Animals of any species entering or exiting the structures were recorded.

An analysis of flood levels in the Macdonald and Gwydir rivers was performed because of flooding damage incurred over the 2 field seasons. Historical mean monthly and maximum monthly discharge data were collected from the NSW Water Real Time Data website for all months between January 2000 and January 2021 from the river monitoring stations at Retreat, NSW (Macdonald River) and Bundarra, NSW (Gwydir River). These monthly mean and monthly maximum data points were sorted into “prior to study period” (January 2020 to August 2019) and “study period” (September 2019 to January 2021) categories and were compared for differences with analysis of variance (ANOVAs). All analyses were conducted in Program R 4.0.3 (R Core Team 2020).

RESULTS

Bell's Turtle Activity. — Female Bell's turtles were recorded in front of the refuge structure entrances in both field seasons (Table 1), and 1 female entered a structure in 2020–2021; however, no nesting activity was directly observed (Fig. 2). No Bell's turtle activity was recorded at Macdonald 2 or Gwydir 1 during either year (Table 1).

Table 1. Activity of Bell's turtles (Myuchelys bellii) and red foxes (Vulpes vulpes) near nesting refuge structures in 2019–2020 and 2020–2021. Individuals could not be distinguished, so the table shows the number of days that the structure was approached by ≥ 1 animal compared with the number of camera monitoring days for each site. NA = not applicable.

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One female was recorded in front of the Macdonald 1 structure on 26 November 2019 from 1723 hrs to 1730 hrs; this female began digging with her hind feet directly in front of the structure entrance (Fig. 2), but left the hole unburied after ∼ 7 min, indicating a possible test dig prior to choosing a final laying site. Between 2 and 3 females were recorded in front of the Gwydir 3 structure on 2 January 2020 from 1617 to 1652 hrs, including 1 female that was recorded digging with her hind feet directly in front of the structure entrance from 1620 to 1652 hrs, but left the hole unburied after 32 min. At 1634 hrs, 2 turtles were recorded in front of the structure, including the one that was digging. A small pit was recorded inside the structure at Gwydir 2 on 27 November 2019, and another directly in front of the structure entrance at Macdonald 3 on 14 December 2019, but the digging activity was not recorded on camera in either instance so could not be assigned to a species (Fig. 2).

One female was recorded inside of the Gwydir 2 structure on 13 November 2020. The turtle was not recorded entering the structure but was seen fully inside the enclosed space at 1158 hrs with its nose close to the soil, possibly searching for a suitable nesting site. The turtle was next recorded leaving the structure at 1205 hrs, without apparently digging any soil inside. However, the female did have some soil on her back (Fig. 2), which may indicate some digging as seen in other turtle species that will flip soil onto their backs during nesting (Harding and Bloomer 1979). One female approached the Gwydir 3 structure on 21 November 2020 at 1439 hrs, remaining stationary in front of the entrance until 1440 hrs before it moved away from the structure entrance. One female was recorded directly in front of the Macdonald 3 structure on 3 December 2020 at 2014 hrs; the image was dark, so the turtle's activity was not discernible. However, a later site visit showed a pit dug in approximately the location where the turtle was photographed, suggesting another test dig.

A cluster (n = 3) of raided turtle nests was located ∼ 5 m from the Gwydir 2 structure on 4 December 2020, ∼ 50 cm from the water's edge. A cluster (n = 5) of raided turtle nests was located ∼ 10 m from the Macdonald 1 structure on 17 December 2020, ∼ 1 m from the water's edge.

Red Fox Activity. — Red foxes were recorded near some structures in 2019–2020, but no fox activity was recorded near any structures in 2020–2021 (Table 1). No foxes were recorded attempting to gain entry to the interiors of any structures, either through the entrance or by digging under. Gwydir 2 had the highest rates of fox activity, with near-nightly recordings of foxes; a later site visit located a fox den ∼ 20 m from the structure. Macdonald 3 also had high levels of fox activity, with foxes passing close to the structures frequently during January 2020. Macdonald 1 had two visits in 2019–2020, although the fox or foxes did not approach close to the structure and appeared to be passing the structure by. Foxes were only recorded in front of the structure entrance at Gwydir 3: in the early morning (0333 hrs) and late evening (2154 hrs) of 4 December 2020, and in the late evening (2330 hrs) of 4 January 2020, where the fox appeared to be sniffing at the site where the female Bell's turtle had been digging 2 d prior. The foxes approached close to the structure entrance, but it is unclear whether they interacted with the electrified wire at all.

Other Species. — A large black animal, speculated to be an Australian raven (Corvus coronoides) was recorded inside Gwydir 2 on 27 November 2019; however, the image was unclear so species identification was not certain. Eastern water dragons were frequently recorded inside the Gwydir 2 structure, but it is unclear whether they were nesting. Australian wood ducks (Chenonetta jubata) and eastern water dragons were frequently recorded entering and leaving the structure at Macdonald 1 in 2020–2021 with no signs of aversion. There was no indication that the water dragons were nesting. Australian magpies (Gymnorhina tibicen) were recorded entering and exiting the structure at Gwydir 3 in 2020–2021 with no signs of aversion.

Flooding. — All six structures were inundated at least once during the 2 yrs of the field season. Macdonald 2 and Gwydir 1 were irreparably damaged in 2019–2020, and Macdonald 1 and Gwydir 3 were both irreparably damaged in 2020–2021. Mean monthly discharge (F_1,251 = 0.6, p = 0.43) and maximum monthly discharge (F_1,251 = 0.3, p = 0.59) did not differ in the Macdonald River between the study period and prior seasons. Mean monthly discharge (F_1,251 = 0.1, p = 0.75) and maximum monthly discharge (F_1,251 = 0.2, p = 0.69) did not differ in the Gwydir River between the study period and prior seasons.

DISCUSSION

A female Bell's turtle did enter 1 structure, and 4 of 6 structures were directly approached by females, but no Bell's turtles were detected nesting inside the structures. Nor were other egg-laying species detected nesting within the structures, including eastern water dragons and eastern longneck turtles, which are sympatric with Bell's turtles. Red foxes were detected near 4 of 6 structures and foxes appeared to investigate the entrance of 1 structure, but they were not detected attempting to enter any structures. Although some female Bell's turtles were detected near the refuge entrances and even digging in front of them, raided turtle nests were found near 2 of the structures. Regardless, the persistent risk of flooding renders nest refuge structures of this design ineffective as tools for Bell's turtle conservation.

Nonetheless, some useful information about Bell's turtle nesting preferences was exposed during this study. Bell's turtles sometimes nest in soil that has been previously disturbed, for example in patches of torn-up soil where trees have been removed (G. Hughes, pers. obs.; P. Spark, pers. comm., August 2018). The use of a rotary hoe to disturb the soil was thus intended to entice the females to nest inside the structure, and 4 of 6 structures did show at least some interest by female Bell's turtles. This may be further evidence that females prefer soil that has been recently turned-over as nesting substrata. Bell's turtles may be reluctant to make use of the nest refuge structures as designed; however, the use of a rotary hoe or similar tools to create enticing nesting habitat may be incorporated into other nest protection methods. Using a rotary hoe to create or enhance nesting substrata prior to the nesting season may serve to entice females to nest inside protected areas. Care should be taken to not overuse this technique, however, because it could increase erosion of riverbanks at these sites, especially with spring flooding events concurrent with nesting season. Used sparingly and with due consideration to the long-term integrity of riverbanks, this technique warrants further investigation; turning patches of soil could be part of an effective nest protection strategy for the future that targets key potential nesting habitat, which also warrants further investigation.

The differing behaviors and habitats of Bell's turtles and diamondback terrapins necessitated some changes from the structure design and placement of Quinn et al. (2015); primarily, Bell's turtles nest much closer to the water than has been observed in terrapins (Burger and Montevecchi 1975; NSW OEH 2014; Cann and Sadlier 2017). Consequently, the structures in this study had to be placed quite close to the river's edge, which placed them at risk of damage or displacement due to flooding. All 6 structures were inundated at least once during the 2 field seasons, 4 of the 6 were irreparably damaged by flooding, and these flood events were not atypical for the Macdonald and Gwydir rivers in summer. Evidently, a rigid, immobile structure is not suitable for long-term deployment for protecting Bell's turtle nests, because placing such structures inside the narrow band of riverbank that Bell's turtles nest in will always place them at risk from flooding.

No foxes were shown to gain entry to the structures; however, it is inconclusive whether the structures as designed could effectively exclude the foxes. Without turtles or other animals nesting inside the structures, foxes would have had little incentive to gain entry. Foxes were only recorded investigating the structure entrances at Gwydir 3, and no interaction with the electric wire was directly recorded. Quinn et al. (2015) reported that raccoons (Procyon lotor) were not able to gain entry to their structures if there was an electric wire across the entrance, and it is unlikely that foxes could have entered through the structure entrance without contacting the wire. It is less clear whether they could have burrowed under the sides to gain entry because there was no evidence that any foxes attempted to do so. Other potential egg predators, such as magpies and ravens, were able to enter the structures without apparent difficulty. Ravens have been observed scaring nesting female Bell's turtles back into the river and then consuming the eggs from the exposed nest (L. Streeting, pers. comm., May 2020), and Spencer (2002) records magpies as frequent nest predators in the lower Murray–Darling catchment. Apparently, the structures as designed would have afforded the turtles little protection from avian predators. However, egg predation by birds has been recorded, but it is unclear how great of a threat it is to the species' persistence, compared with the likely threat posed by foxes (NSW OEH 2014).

In conclusion, although the nest refuge structures were recorded with some female Bell's turtles in the local area, the use of such structures in a conservation strategy for this species in particular would most likely be ineffective. This study showed that, although structures of this type may be effective in protecting nests of some turtle species (Quinn et al. 2015; A. Santoro, pers. comm., January 2020), it is not effective for protecting Bell's turtle nests given the species' habitat and apparent reluctance to nest within the structures. It is conceivable that an alternative nesting refuge structure could be designed such that Bell's turtle females would be less averse to using them, but the danger of damage to the structures from flooding will always be present given the preference of this species to nest so close to the edge of river systems.

The importance of negative results in research is often overlooked but are necessary to consider when testing new methods or techniques, particularly in conservation studies (Axford et al. 2020). Conservation organizations have limited resources; and investing these resources in an ineffective strategy when more effective alternatives may be available is an error to be avoided. Other avenues for protecting Bell's turtle nests should be investigated in the future, such as conditioned food aversion, which has been shown to reduce nest predation by foxes on ground nesting birds (Tobajas et al. 2020). Investigation into the proximate and ultimate questions underpinning the choice of Bell's turtles to nest on riverbanks highly prone to flooding may yield interesting evolutionary adaptations or may be evidence for an ethological trap similar to that described by Spencer et al. (2016) in the context of predation, if flooding has become more intense and less predictable since colonization. Instead of nesting refuge structures, alternative strategies such as conditioned food aversion could be potentially combined with turning over soil to entice Bell's turtles to nest in protected nesting areas, which may prove more effective than the use of a refuge structure while still enabling managers to devote less time to protecting individual nests, and thereby to operate more efficiently over a breeding season.