Protect nests and nesting sites from predation using artificial nest covers: Sea turtles
Overall effectiveness category Awaiting assessment
Number of studies: 15
Background information and definitions
Using temporary individual nest covers may be a preferred way of protecting reptile nests when predator removal is difficult (for example due to the predator’s ecology), or when it is considered to be unethical (for example involving controls on endangered species), unpopular (for example charismatic or native predators) or too costly (Buzuleciu et al. 2015).
Due to the number of studies found, this action has been split by species group, though no studies were found for amphisbaenians.
For studies that use fencing to exclude predators from larger areas, see Remove or control predators using fencing and/or aerial nets.
Buzuleciu S.A., Spencer M.E. & Parker S.L. (2015) Predator exclusion cage for turtle nests: a novel design. Chelonian Conservation and Biology, 14, 196–201.
Supporting evidence from individual studies
A replicated, randomized, controlled study in 1993–1994 on a long sandy beach in Florida, USA (Ratnaswamy et al. 1997) found that covering sea turtle nests with a wire screen resulted in lower nest predation compared to when no screen was used. Predation of screened nests (9% and 7% of 499 and 737 nests predated) was lower than for nests with no screen (31% and 24% of 231 and 379 nests predated). An additional group of nests that received both a screen and a conditioned taste aversion treatment had similar predation to both screened nests and non-screened nests (16% and 12% of 531 and 720 nests predated). In 1993–1994, a long stretch of barrier beach (37 km) was broken down into four experimental blocks, and around 2.5 km of each block was selected for a screening trial, with two-thirds of nests in the area receiving a screen (1.2 x 1.2 m wire screen, with 5.1 x 10.2 cm mesh). Screens were secured with steel rebar at each corner. Nests in the remainder of the block either received no treatment or were part of further tests of the effect of conditioned taste aversion or raccoon Procyon lotor removal. Two-thirds of nests in the taste aversion area were also screened. Taste aversion involved provision of chicken eggs injected with 10 mg of oral oestrogen. Turtle nests were monitored 2–4 times/month in 1993 and 2–3 times/week in 1994.Study and other actions tested
A randomized, controlled study in 1992 on a sandy beach in Turkey (Yerli et al. 1997) found that protecting loggerhead turtle Caretta caretta nests with wire mesh grids reduced fox Vulpes vulpes predation and increased hatching success compared to unprotected nests. No protected nests were predated (0 of 25 nests) whereas 63% of unprotected nests experienced at least one predation event (55 of 88 nests). Protected nests had increased hatching success compared to unprotected nests (data reported as statistical model outputs). Hatching success at nine nests that were predated before protection was greater than at nests without any protection (data reported as statistical model outputs). There was evidence of fox activity (such as digging) at eight protected nests, but the predation attempts were unsuccessful. A sandy beach (4.7 km long) was patrolled in June-August 1992 for signs of turtle nesting and fox activity. In total 25 nests were selected randomly and covered with 1m2 mesh grids positioned 5 cm under the sand, directly above the eggs as soon as possible after laying. Nine of the 25 nests had already been predated once before the cover was put in place. Covered nests and 88 nests without covers were monitored daily for signs of further predation (also digging and defecation) and hatching success (the number of emerging hatchlings) for 30 consecutive nights.Study and other actions tested
A replicated, controlled, paired study in 1996 on a beach in Florida, USA (Mroziak et al. 2000) found that loggerhead Caretta caretta turtle nests covered with cages had higher predation rates compared to uncovered nests. Caged sea turtle nests had higher predation rates (high predation beach sections: 42–47% nests predated, low predation beach sections: 15–22%) compared to uncaged nests (3–4% and 0–3% respectively). Approximately one third of predated nests were completely destroyed whether nests were caged (11 of 29 nests) or uncaged (1 of 3 nests). Decoy cages (with no nests) were predated in high and low predation beach sections (high: 10 of 18 nests, low: 6 of 14 nests), in some cases multiple times (high: 7 of 10 nests predated >once, low: 2). Hatchling numbers from unpredated caged or uncaged nests were similar (80 hatchlings/nest) and higher than hatchling numbers from partially predated nests (50 hatchlings). Racoons Procyon lotor caused 88% of predation, grey foxes Urocyon cinereoargenteus 11% and spotted skunks Spilogale putorius 1%. In May–October 1996, sea turtle nests were covered with square wire cages (76 cm square, 107 cm tall, 5 x 10 cm mesh) anchored 30 cm in the sand or left uncovered in pairs in two low (40 nest pairs) and two high predation beach sections (50 nest pairs). Nest pairs were laid within 2 days of each other and in their natural position (<15 m apart, 53 pairs) or relocated to create a pair (>4 m apart, 37 pairs). Thirty-two decoy cages not covering actual nests were placed on the beach (high predation: 18 cages; low predation: 14). All nests (including decoys) were checked daily for signs of predation until October and the likely identity of the predator. Nests were excavated three days after emergence to count successfully hatched eggs.Study and other actions tested
A replicated, controlled study in 2001–2002 on two sandy beaches in southwest Turkey (Başkale & Kaska 2005) found that covering sea turtle nests with screens resulted in fewer eggs being predated and higher overall hatching success compared to nests that were not screened. Results were not statistically tested. Fewer nests with screens were predated (0 of 54 nests) compared to unscreened nests (Dalaman beech: Of 49 nests, 20 partially predated, 13 entirely predated, 888 eggs predated; Dalyan beach: Of 40 nests, 29 predated, 2,200 eggs predated). Overall hatching success was higher for nests with screens (screened: 74%; unscreened: 54%). Beaches were searched for nests, and those at risk of predation (54 nests on Dalaman beach) were screened with a metal grid (72 x 72 cm) and a 9 cm mesh buried 20 cm deep. A further 89 nests (49 on Dalaman beach; 40 on Dalyan beach) received no screen. Nests were monitored from June–September in 2001 (Dalyan beach) and 2002 (Dalaman beach).Study and other actions tested
A replicated, controlled study in 1987–1995 on a sandy beach on Zakynthos Island, Greece (Kornaraki et al. 2006) found that covering loggerhead turtles Caretta caretta nests with metal cages resulted in variable hatching success compared to nests left in situ and nests relocated to an on-beach hatchery. Hatching success for caged nests varied from 44% to 72%, compared to 56–68% for in situ nests and 51–75% for hatchery nests. Hatching success in caged nests was lower in two of six years and higher in two of six years compared to in situ nests. From 1988, nests located within 7 m of the sea and in danger of inundation were moved to a beach hatchery (77 nests) as were nests located near invasive plants which had root systems that could grow into nests. From 1990, nests located in beach areas with tourists were protected by 50 cm circular metal mesh cages buried 15 cm in the sand (88 nests). A further 313 nests were left in situ. Nests were excavated following hatchling emergence to assess hatching success.Study and other actions tested
A replicated, controlled study in 2005 on one beach in northeast Qatar (Ficetola 2008) reported that covering hawksbill turtle Eretmochelys imbricata nests with plastic nets resulted in less predation by feral cats Felis catus and Ruppell’s foxes Vulpes rueppelli compared to when nests were not covered. Zero of 16 nests covered with plastic nets were predated, whereas all nests that were not covered were either partially predated (6 of 31, 19%) or completely predated (25 of 31, 81%). Before plastic nets were deployed a further three nests were partially predated, and one was completely predated. In July 2005, sixteen nests were covered with plastic nets, and 35 were left uncovered. In April–September 2005, a 1.4 km stretch of beach was patrolled five times/day, and a further 1.7 km stretch was patrolled every 7–10 days. After four nests in the intensively searched stretch of beach were predated, all nests in this stretch of beach were covered with plastic nets.Study and other actions tested
A replicated, controlled study in 2010 on a sandy beach in North Carolina, USA (Kurz et al. 2012) found that both metal cages and plastic screens reduced predation of artificial sea turtle nests compared to nests with no protection. Artificial nests protected with metal cages and plastic mesh were predated less by red foxes Vulpes vulpes (metal cage: 0 of 12 predated; plastic mesh: 0 of 12 predated) than nests with no protection (4 of 12 predated). In an additional experiment (high predator motivation), a similar number of nests protected by plastic mesh (2 of 8) were predated compared to nests protected with metal cages (0 of 8). Artificial nests consisting of five chicken eggs and scented with dilute loggerhead egg yolk mixture were buried 29 cm deep. Twelve were protected with a metal cage (122 x 61 x 61 cm, buried 30 cm deep), 12 with a plastic mesh (2.4 m2 centred on nest), and 12 were unprotected. In an additional experiment (high predator motivation), nests consisted of bacon and rotten chicken eggs or chicken breast and bacon scraps, and eight pairs of nests were protected with a metal cage (8 nests) or plastic mesh (8 nests). Nests were checked daily for signs of predation.Study and other actions tested
A randomized, controlled study in 2009–2010 on a sandy beach in East Java, Indonesia (Maulany et al. 2012) found that using artificial nest covers to protect olive ridley turtle Lepidochelys olivacea nests did not improve hatching success rates. All nests, including those with artificial covers and those without were predated within one week of being laid and no eggs hatched. Olive ridley turtle nests laid in May–July 2009–2010 along an 18 km stretch of sandy beach in a national park were randomly selected to be either protected by artificial nest covers (2009: 5; 2010: 5) or unprotected (2009: 6; 2010: 14). Nests were excavated to count the number of eggs and re-buried. Protected nests were covered with a 40 x 50 x 50 cm cylindrical galvanized wire cage buried 20 cm into the sand and secured with wooden stakes. All nests were temporarily covered prior to hatching to enable hatchlings to be counted. After emergence, all nests were dug up and unhatched eggs counted.Study and other actions tested
A replicated, controlled study in 2010 on a sandy beach in South Carolina, USA (Lamarre-DeJesus & Griffin 2013) found that using screens to cover loggerhead turtle Caretta caretta nests resulted in reduced predation by coyotes Canis latrans compared to nests with no protection. The number of predated nests was lower for screened nests (7 of 33, 21%) compared to those with no cover (6 of 10, 60%). A similar number of screened nests were predated compared to nests covered with pepper powder on the surface (2 of 10, 20%), but nests with pepper powder below the surface had similar predation as those with no treatment (5 of 10, 50%). Thirty-three nests were covered with a plastic or metal screen (1 x 1 m), and 10 were given no screen. A further 10 nests were covered with 15 ml of habanero pepper Capsicum chinense powder on the surface of the nest, and 10 with pepper powder below the surface. In June–July 2010, nests were monitored for complete or partial predation every 1–3 days, and a further 12 visits were made until September.Study and other actions tested
A controlled study in 2008 on a sandy beach in Boa Viste, Cape Verde (Marco et al. 2015) found that loggerhead turtle Caretta caretta nests reburied in mesh cages or under netting had higher hatching success and lower rates of ghost crab Ocypode cursor predation compared to unprotected nests. Hatching rates were higher in nests that were protected with mesh (cage: 82%; netting: 60% success) compared to nests that were not protected (33% success). Ghost crab predation rates were lowest in nests that were buried in mesh cages (4%), compared to under mesh netting (22%) or unprotected nests (55%). Turtle nests were excavated, eggs counted and reburied in the same place either inside a mesh cage (20 nests), underneath a horizontal 1m2 plastic mesh buried 10 cm under the surface (20 nests) or without any protection (20 nests). Nests were monitored daily until emergence. Hatchlings were counted and released from nests with protection. Hatchling tracks were counted from nests with no protection. All nests were excavated after last emergence and remaining eggs counted for analysis.Study and other actions tested
A replicated, randomized, controlled study in 2002–2007 on two sandy beaches in Georgia, USA (McElroy et al. 2015) found that using plastic mesh screens to cover loggerhead turtle Caretta caretta nests resulted in no predation and similar hatching and emergence success compared to nests with no covers. No nests covered with plastic mesh were predated, whereas nine nests with no cover were fully predated, and nine were partially predated (result were not statistically tested). Hatching and emergence success was similar for nests with covers (hatching: 73–76%; emergence: 67–68%) and without covers (hatching: 70–80%; emergence: 67–78%). Two stretches of beach (3 and 7 km) were searched daily during May–October 2002–2007. Nests were either covered with a 1 m2 plastic mesh screen (85 relocated to nearby dune; 75 left in situ) or received no screen (83 relocated; 137 left in situ). Nests were monitored daily for predator activity, and five days after hatchling emergence began, nests were excavated, and the numbers of hatched and unhatched eggs and live or dead hatchlings were counted.Study and other actions tested
A study in 2005–2010 on a sandy beach on an island off the coast of Florida, USA (Engeman et al. 2016) found that using nest cages to cover sea turtle (loggerhead Caretta caretta and green Chelonia mydas) nests reduced predation by raccoons Procyon lotor but not by feral pigs Sus scrofa. Covering marine turtle nests with cages reduced nest predation by raccoons in five of six years (1–20% of nests predated) compared to uncaged nests (7–69% of nests predated). Cages did not prevent feral pigs from predating nests. In August 2007, 36 of 36 remaining unhatched caged nests and 14 of 14 remaining unhatched uncaged nests were predated by pigs. Caged nests took longer to be predated by pigs (20 days) compared to uncaged nests (8.5 days). Turtle nests were monitored daily throughout the nesting season on a 13 km long stretch of beach on an island (526 ha) in 2005–2010. Located nests were covered by partially buried cages (91 cm long x 91 cm wide x 76 cm tall, 5 x 10 cm wire mesh) to protect them from raccoon predation (54–159 nests/year were caged, 8–24 nests/year were uncaged). After hatching, all nests are excavated to record hatching success and predation levels. In 2007, nest predation by pigs was observed for the first time (pigs were present on the island from 2001). Pigs were eradicated from the island in 2008 but reinvaded in 2014. See ‘Use lethal controls’ for more details.Study and other actions tested
A replicated, controlled study in 2014–2016 at one beach in south-eastern Queensland, Australia (Lei & Booth 2017) found that using aluminium cages or plastic mesh to cover loggerhead turtle Caretta caretta nests from yellow-spotted goanna Varanus panoptes predation led to lower nest predation in one of two years. In 2014–2015, predation was lower in nests covered in plastic mesh (2 out of 11) and aluminium cages (5 out of 10), compared to nests with no covering (10 out of 11). In 2015–2016, predation did not differ significantly between nests covered in plastic mesh (0 out of 15) and nests with no covering (2 out of 16). In May 2014–June 2015, ten nests were covered with aluminium cages, 11 with plastic mesh, and 11 nests had no covering. In June–July 2015–2016, fifteen nests were covered with plastic mesh and 16 nests had no covering. Aluminium or plastic covers were buried over the top of the nest at a depth of 10–20 cm. Each nest was visited daily in early December to the end of February 2014–2016 to record predation events.Study and other actions tested
A replicated, controlled study in 2005–2014 on eight mostly connected beaches in Queensland, Australia (O’Connor et al. 2017) found that sea turtle nests covered with mesh were predated less frequently than those not covered during five years when fox Vulpes vulpes control was being carried out, but a similar amount during the following five years when foxes were not controlled. In 2005–2009 when foxes were being controlled, nests covered with mesh were predated less frequently (4–28% of 18–56 nests) than those without mesh (43–100% of 7–13 nests). In 2010–2014 when foxes were not controlled, overall predation of nests was very low, and a similar number of covered (0–4% of 25–51 nests) and uncovered nests were predated (0–25% of 0–5 nests). In 2005–2014, meshing (plastic or aluminium) was used to cover all sea turtle nests that were discovered (18–56 nests/year). A number of other nests were not discovered until after hatching and so were not covered with mesh (0–13 nests/year). In 2005–2009, a total of 19 foxes were trapped and euthanized, and a number of fox dens were fumigated (number not given). No formal fox control occurred in 2010–2014, though three foxes were removed from the area for unrelated reasons. Nests were monitored continuously throughout November–April and predation of nests by foxes was recorded.Study and other actions tested
A replicated, controlled study in 2014–2015 on one sandy beach on the Caribbean coast of Costa Rica (Pheasey et al. 2018) found that using a screen to cover green turtle Chelonia mydas and hawksbill Eretmochelys imbricata nests resulted in a similar number of predation attempts by domestic dogs Canis lupus familiaris compared to when no screen was used, but the success of predation attempts varied depending on screen type. The number of predation attempts was similar for nests covered with plastic screens compared to those without screens (data reported as statistical model results). The number of successful predation attempts depended on whether a bamboo screen (16 of 31, 52%), a plastic screen (38 of 47, 81%) or no screen (31 of 31, 100%) was used to cover the nest. Fewer predation attempts were made on nests when screens were deployed just after eggs were laid (74% of nests) compared to just before hatchlings emerged (97% of nests) (number/treatment not reported), though the likelihood of successful predation did not differ (data reported as statistical model result). In March-October 2014–2015, a total of 227 nests were either covered with a plastic or bamboo screen or were left with no screen (number/treatment not reported). Screens were buried over the top of nests at a depth of 25–30 cm for green turtle nests and 10 cm for hawksbill nests. All nests were checked for predation attempts daily during the whole incubation period.Study and other actions tested