Protect nests and nesting sites from predation using artificial nest covers: Tortoises, terrapins, side-necked & softshell turtles

Supporting evidence from individual studies

1. A replicated, site comparison study in 1964–1972 on three islands in the Galápagos, Ecuador (MacFarland et al. 1974) found that protecting Galápagos giant tortoise Geochelone elephantopus nests with rock-walled corrals reduced predation by feral pigs Sus scrofa. In 1964–1970, none of the giant tortoise nests (10–25/year) on one island protected using corrals were predated by pigs. In 1970–1972, one of 262 protected nests were predated by pigs compared to 23 of 29 unprotected nests. The authors reported that corrals did not prevent dogs Canis lupus familiaris accessing and destroying nests elsewhere. In the 1964/1965–1969/1970 nesting seasons, 10–25 giant tortoise Geochelone elephantopus porteri nests/year were protected from pig predation on Santa Cruz using corrals built with lava-rock walls (1.5–2 m diameter, 1 m high). Corral use was extended in the 1970/1971 and 1971/1972 nesting seasons to protect 262 nests of three subspecies of giant tortoise (G. e. porteri, G. e. vicina, G. e. darwini) on three islands (Santa Cruz, San Salvador and Isabela). In 1970, twenty-nine nests were not protected with corrals on Santa Cruz and hatching outcomes monitored. In 1971–1972, introduced mammals (pigs and goats Capra hircus) were also controlled by shooting.

   Study and other actions tested

   Referenced paper

   MacFarland C.G., Villa J. & Toro B. (1974) The Galápagos giant tortoises (Geochelone elephantopus) Part II: Conservation methods. Biological Conservation, 6, 198-212.

2. A replicated, randomized, controlled study in 2010–2011 at two lakes within mixed forest in Ontario, Canada (Riley & Litzgus 2013) found that covering painted turtle Chrysemys picta and snapping turtle Chelydra serpentine nests with one of three different cages did not affect hatching success compared to uncaged nests, and that cage design did not affect the number of predator interactions or successful predation events. For both species, hatching success was similar for caged (painted turtle: 69–79%; snapping turtle: 73–85%) and uncaged nests (painted turtle: 60%; snapping turtle: 73%). Comparisons of three cage types (uncaged nests not included) found that there was no significant difference between the number of predator interactions (above-ground cages: 14; below-ground: 16; wooden cage: 2) and successful predation attempts (above-ground cages: 3; below-ground: 1; wooden cage: 3). Nesting sites were monitored in May–June 2010 and June–July 2011. Nests were excavated and assigned to one of four treatments: above-ground wire cage (50 nests); below-ground wire cage (49 nests); above-ground wooden cage (24 nests); or no nest covering (41 nests). Wooden cages were used only in 2011. Predator interactions and successful predations were recorded throughout the nesting season and after hatchling emergence all hatchlings and unhatched eggs were counted.

   Study and other actions tested

   Referenced paper

   Riley J.L. & Litzgus J.D. (2013) Evaluation of predator-exclusion cages used in turtle conservation: Cost analysis and effects on nest environment and proxies of hatchling fitness. Wildlife Research, 40, 499-511.

3. A replicated, controlled study in 2014 in one brackish wetland in New York, USA (Burke et al. 2015) found that diamondback terrapin Malaclemys terrapin nests that were covered with metal cages had high hatching success, and nests with no cages suffered very high levels of predation. Hatching success of nests covered with cages ranged from 55–93% (number predated not given). Nests with no cages suffered high levels of predation (15 of 18 to 15 of 15, 83–100% of nests; hatching success not given). In addition, the application of pepper powder to caged nests had no effect on hatching success (pepper: 55–93%; no pepper: 78–83%). In June–July freshly laid nests were located and 11 were covered with metal mesh cages (15 cm deep). Two nests were covered with 10 g of pepper powder, and 9 received no pepper. A further 48 nests received no cages, though 15 were covered with 10 g of pepper, and 15 with 20 g of pepper. All nests were monitored daily: uncaged nests for a minimum of seven days, and caged nests until mid-November, at which point they were excavated to determine hatching success.

   Study and other actions tested

   Referenced paper

   Burke R.L., Vargas M. & Kanonik A. (2015) Pursuing pepper protection: habanero pepper powder does not reduce raccoon predation of terrapin nests. Chelonian Conservation and Biology, 14, 201-203.

4. A replicated, controlled study in 2013 in three estuarine sites in South Carolina, USA (Buzuleciu et al. 2015) found that covering artificial turtle nests with one of three cage designs resulted in less predation by raccoons Procyon lotor compared to when no nest cover was used. In a comparison between three cage designs, the “birdcage” design was more effective at preventing predation (0 of 4 nests predated) than a metal cage, (2 of 4) plastic cage (4 of 4) or no cage (4 of 4). Two further trials with the “birdcage” design found that artificial nests covered with the cage were predated less than nests with no cage (cage: 0 of 8, 100% and 25 of 84, 30% predated; no cage: 8 of 8, 100% and 71 of 84, 85%). Sixteen simulated nests were created at a diamondback terrapin Malaclemys terrapin nesting site by digging and immediately refilling a nest-sized hole. Three cage designs were used to cover four nests each, and four nests received no cover. Two further trials tested the “birdcage” design against no nest cover (trial 1: 8 caged, 8 un-caged; trail 2: 84 caged, 84 un-caged). Artificial nests were left for 48 hours and predation attempts were recorded.

   Study and other actions tested

   Referenced paper

   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.

5. A replicated, before-and-after study in 2013–2014 on an island site between a saltmarsh and road in Georgia, USA (Quinn et al. 2015) found that electrified nest boxes provided more protection for diamondback terrapin Malaciemys terrapin nests from predation than a nest box alone. Fewer nests laid under nest boxes with an electric wire were predated (1 of 27 nests found) compared to those under nest boxes with no wire (16 of 16 nests found). Nests laid on the artificial nest mound yielded at least 203 hatchlings. An artificial nesting mound (22.9 m long × 3.6 m wide × 1.2 m tall) was constructed using dredge material along the shoulder of an 8.7 km causeway leading to the island. On top of the mound were placed six nest boxes (3.7 x 1.2 x 0.6 m) with a ground-level 9 cm horizontal gap to allow terrapins access but to exclude predators. For 35 days from May–June 2013, one nest box was modified to include a battery-powered electric wire along the horizontal gap opening and for 26 days from June–July 2013, all six nest boxes had electric wires. The mound was excavated to find nests and hatched eggs in November 2013 and April 2014.

   Study and other actions tested

   Referenced paper

   Quinn D.P., Kaylor S.M., Norton T.M. & Buhlmann K.A. (2015) Nesting mounds with protective boxes and an electric wire as tools to mitigate diamond-backed terrapin (Malaclemys terrapin) nest predation. Herpetological Conservation and Biology, 10, 969-977.

6. A replicated, controlled study in 1974–2012 in 11 wetland sites in New Jersey and Pennsylvania, USA (Zappalorti et al. 2017) found that caged bog turtle Glyptemys muhlenbergii nests had lower predation rates compared to uncaged nests, but overall hatching success was not higher. Fewer eggs were predated in caged nests (6 of 97 eggs, 6%) compared to uncaged nests (82 of 161 eggs, 51%), but overall hatching success was not higher for caged nests (caged: 42 of 97 eggs, 43%; un-caged: 53 of 161 eggs, 33%). Cages of 1 cm wire mesh were installed over nests (61 cm high, 38 cm wide) and buried 8–15 cm into the ground. In June 1974–2012, twenty-seven nests in five wetlands were covered with cages, and 55 nests in 11 wetlands were left uncaged. Eggs were monitored for at least 8–9 weeks to record predation and hatching success.

   Study and other actions tested

   Referenced paper

   Zappalorti R.T., Tutterow A.M., Pittman S.E. & Lovich J.E. (2017) Hatching Success and Predation of Bog Turtle (Glyptemys muhlenbergii) Eggs in New Jersey and Pennsylvania. Chelonian Conservation and Biology, 16, 194-202.

7. A before-and-after study in 2008–2016 in wet meadow, marsh and fen habitat in New York, USA (Travis et al. 2018) found that when artificial nest covers were used to protect bog turtle Glyptemys muhlenbergii nests in an area that was also grazed by cattle, a higher proportion of eggs hatched compared to when there was no grazing and no nest covers were used. Results were not statistically tested. In four years when bog turtle nests were protected by artificial covers in an area grazed by cattle, overall hatching success was 52% (58 of 112 eggs hatched). When nests were not protected and there was no grazing overall hatching success over two years was 27% (4 of 15 eggs hatched). The authors reported that the nest covers protected nests from larger predators such as raccoons Procyon lotor, but not from smaller, burrowing predators. In 2012–2016, bog turtle nests (3–12 nests/year, 15–47 eggs/year) in a fenced wetland being grazed by cattle (5.6 ha) were protected by mesh cloth artificial nest covers (12 x 12 x 12 cm) held in place by metal pegs. In 2014, some nests were predated before covers were put in place, so 2014 results are not included here. In 2009–2010, prior to grazing being introduced, bog turtle nests (2–3 nests/year, 7–8 eggs/year) with no nest protection were monitored. Nests were located by surveying on foot in 2009–2016.

   Study and other actions tested

   Referenced paper

   Travis K.B., Kiviat E., Tesauro J., Stickle L., Fadden M., Steckler V. & Lukas L. (2018) Grazing for bog turtle (Glyptemys muhlenbergii) habitat management: case study of a New York fen. Herpetological Conservation and Biology, 13, 726-742.