Action: Physically protect nests with individual exclosures/barriers or provide shelters for chicks of waders
- Three of 13 studies from the USA found higher productivity from nests protected by individual barriers than unprotected nests. Two studies from the USA and Sweden found no higher productivity from protected nests.
- Eight studies from the USA and Europe found higher hatching rates, or survival, or low predation of nests protected by individual barriers, although two of these found that higher hatching rates did not result in higher productivity. Two small studies from North America found no differences in predation or survival rates between protected and unprotected nests.
- A meta-analysis from the USA found that there were differences in the effectiveness of different exclosure designs.
If fencing does not work to exclude predators (for example, predatory birds), or is not a viable option, it may be possible to protect individual nests using a variety of cages and exclosures. These must be able to allow chicks and adults to get in and out, but not predators and should be quick to install to minimise the chances of parents abandoning nests (see ‘Can nest protection increase nest abandonment?’).
Unfortunately, because each cage is over a nest, it is possible that predators will learn the association and that providing the exclosures will actually increase predation on either adults or chicks (see ‘Can nest protection increase predation of adult and chick waders?’)
The effectiveness score for this intervention included a combined assessment of the evidence for chicks of ground nesting seabirds, chicks of waders, chicks of storks and ibises, and chicks of songbirds.
Supporting evidence from individual studies
A small randomised, replicated and controlled trial in 1978 on a beach on Lake Erie, Canada (Nol & Brooks 1982) found that predation rates of killdeer Charadrius vociferous nests were not significantly different between 12 nests protected with a novel predator exclosure (‘H’ shaped frame with the nest in the centre and eight 7 x 12 cm openings, covered in 1.4 cm mesh hardware cloth) and 17 control nests (75% of protected nests and 71% of unprotected nests predated). However, no protected nests were lost to avian predators (gulls Larus spp. or American crow Corvus brachyrhynchos). Instead they were predated by raccoons Procyon lotor or mustelids which could enter exclosures.
A 1992 meta-analysis (Deblinger et al. 1992) analysed data from 211 nest exclosures across eight US states and three Canadian provinces to determine exclosure features that led to lowest predation rates of piping plover Charadrius melodus nests. Overall, exclosures were effective (10% (21) of nests being predated, mainly by red foxes Vulpes vulpes but also American crows Corvus brachyrhynchos and other predators). Estimated predation probabilities revealed that: mid-sized exclosures (3-6 m2) suffered higher predation (26% of 48 exclosures) than small (<3 m2, 5% of 23) or large (>6 m2, 8% of 140) exclosures. Square enclosures were predated at a higher rate (72% of 19) than circular (8% of 166) or triangular (0% of 26) ones. Exclosures supported by ‘tomato stakes’ (thin gardening stakes) were predated more (80% predation of 18 exclosures) than unsupported (3% of 35) or metal/wood supported (8% of 158). Exclosures with mid-height posts (122 cm) were predated more (29% of 40) than short (<122 cm, 3% of 41) or tall (>122, 9% of 130) posts and exclosure with low fences (<122 cm, 42% of 27) were more likely to be predated than those with high (>122 cm, 8% of 184) fences. Fences buried to less than 10 cm were more likely (27% of 62) to be predated than those buried to more than 10 cm (6% of 149). There were no significant differences between different mesh sizes (5 x 10 cm vs. 5 x 5 cm) or whether exclosures were covered or not.
A replicated, controlled before-and-after study from 1988-1989 at six beaches on Cape Cod, Massachusetts, USA (Melvin et al. 1992), found that the daily survival rates, overall hatching success and number of chicks fledged/pair of 29 piping plover Charadrius melodus nests protected by wire predator exclosures (circular wire fences, 1 m tall, around nests with 5 x 10 cm mesh) were significantly higher than for 24 unprotected nests (daily survival rates: 99% for protected vs. 93% and unprotected nests; overall hatching success: 74% of 104 eggs vs. 19% of 59 eggs; 1.96 chicks/pair vs. 0.12 chicks/pair). Before exclosures were used, between 1985 and 1987, 79% of 126 plover nests were destroyed by predators (red foxes Vulpes vulpes, striped skunks Mephitis mephitis, American crows Corvus brachyrhynchos and gulls Larus spp.).
A controlled, replicated study from 1986-1989 on a beach in Massachusetts, USA (Rimmer & Deblinger 1992) found that hatching rates of 26 piping plover Charadrius melodus nests protected by triangular wire mesh fences (5 cm wire mesh, 30.5 m perimeter, placed around individual nests) were higher than for unprotected nests (92% and 25% of nests hatching at least one egg, respectively). On average, protected nests produced significantly more nestlings than unprotected nests (3.5 and 1 chicks/nest respectively). All but one of the losses of unprotected nests was due to predation; no protected nests were predated.
A replicated, controlled study in 1992 in the North Slope of Alaska, USA (Estelle et al.1996) found that the average daily survival rate of 13 pectoral sandpiper Calidris melanotos nests protected by wire mesh cages was significantly higher than that for 39 unprotected nests (survival rates of 0.98 for caged and 0.72 for uncaged nests). The mesh of the 31 cm tall and 66-69 cm diameter cages was sufficiently large (5 x 10 cm) to allow female plovers to enter and exit, but prevented arctic foxes Alopex lagopus from digging under or entering nests. No protected nests were lost to predation.
A small replicated, controlled study from 1994-95 on beaches and alkaline flats in Colorado, USA (Mabee & Estelle 2000) found that daily survival rates of 27 snowy plover Charadrius alexandrinus, 16 killdeer C. vociferous and 9 piping plover C. melodus nests (in 1994) and 28 snowy plover nests (in 1995) were no higher for nests protected by predator exclosures (61 cm tall, 122 cm in diameter cylinders of 5 x 5 cm or 5 x 10 cm wire mesh, for snowy and piping plover or killdeer respectively) than for unprotected nests (daily survival rates in 1994: 0.98 vs. 0.98 for snowy plovers; 0.97 vs. 0.99 for killdeer; 0.98 vs. 0.98 for piping plovers; in 1995: 0.98 vs. 0.97 for snowy plovers). Five protected nests (20%) were predated in 1994 and three (21%) in 1995, by snakes, rodents or skinks. The authors cite small sample sizes, unbalanced experimental design and ‘inappropriate statistical analyses’ as possible reasons for the lack of a significant result.
A replicated, controlled study from 1996-1997 at three alkali lakes in North Dakota and Montana, USA (Murphy et al. 2003) found that the average number of fledglings produced by piping plover Charadrius melodus pairs provided with fences (0.9 m tall and made from 5 cm poultry wire) around individual nests was significantly higher than for unprotected pairs (1.7 chicks/pair for 46 pairs with nest fences vs. 0.7 chicks/pair for 43 unprotected nests). This study is described further in ‘Use multiple barriers to protect nests’.
A replicated before-and-after study from 1984-90 and 1991-99 on beaches in California, USA (Neuman et al. 2004) found that the mean hatching rate and hatching rate/male of snowy plover Charadrius alexandrinus nests increased following the protection of nests with 1.5 m high wire exclosures (hatching rate in 1984-90: 43% of 728 nests vs. 68% of 682 nests in 1991-99; hatching rate/male: 2 chicks/male vs. 2.7 chicks/male). However, the mean number of chicks fledged per male did not change (0.86 and 0.81 chicks fledged/male for 1984-90 and 1991-99 respectively). Between 1993 and 1999, there was also the targeted removal of red foxes Vulpes vulpes and feral cats Felis catus, described in ‘Control predators not on islands’. The study also discusses nest abandonment and adult mortality, see ‘Can nest protection increase nest abandonment?’ and ‘Can nest protection increase predation of adults and chicks?’.
A small, replicated, before-and-after study at a site in eastern England (Gulickx & Kemp 2007) found that the average productivity of little ringed plover Charadrius dubius increased from 0.6 chicks/pair during 1984–95 to 1.6 chicks/pair during 1996–2005 following the protection of nests with wire cages (61 x 61 x 30.5 cm cages with 5 x 5 cm mesh). The average number of nesting pairs at the site increased from 1.3 pairs/year prior to nest protection to 7.6 pairs/year after protection.
A small trial in eastern England (Gulickx et al. 2007) found that neither of two little ringed plover Charadrius dubius nests that were protected by wire cages (61 x 61 x 30.5 cm cages with 5 x 5 cm mesh) in 2005 and 2006 were lost to predation.
A replicated, randomised and controlled trial in 2002 and 2004 at three grazed pasture sites in south-west Sweden (Isaksson et al. 2007) found that nests provided cages (truncated cone steel cages with 6.5 – 8.5 cm spacings between vertical bars and 4 x 4 cm steel netting on top) had significantly higher average daily survival rates than unprotected nests in both common redshank Tringa totanus (99.7% for 34 protected nests vs. 96% for 32 unprotected nests in 2002) and northern lapwing Vanellus vanellus (99% for 37 protected nests vs. 97 for 153 unprotected nests in 2002 and 2004). However, there was higher predation of adult redshank on protected nests, and possibly higher abandonment by lapwings, see ‘Can nest protection increase predation of adults and chicks?’.
A replicated, controlled before-and-after study from 1999-2004 on pastures in southwest Sweden (Pauliny et al. 2008) found that the average hatching rate of southern dunlin Calidris alpina schinzii nests was significantly higher for nests protected by steel cages (20 cm high truncated cones with 7.5 cm gaps between vertical bars and 4 x 4 cm steel mesh covering the top) than for unprotected nests (67% of 25 protected nests survived to hatching vs. 41% of 61 unprotected nests). Moreover, protected nests were more likely to hatch more than one chick (80% of 25 protected nests vs. 57% of 60 unprotected nests). However, comparing 1993-98 (when no nests were protected) with 1999-2004 (when some nests were protected) revealed that there was no significant change in either the number of fledglings/breeding adult or the number of new recruits/breeding adult produced by the study sites. This study is also discussed in ‘Can nest protection increase predation of adults and chicks’?
- Nol E. & Brooks R.J. (1982) Effects of predator exclosures on nesting success of killdeer. Journal of Field Ornithology, 53, 263-268
- Deblinger R.D., Vaske J.J. & Rimmer D.W. (1992) An evaluation of different predator exclosures used to protect Atlantic coast piping plover nests. Wildlife Society Bulletin, 20, 274-279
- Melvin S.M., MacIvor L.H. & Griffin C.R. (1992) Predator exclosures: a technique to reduce predation at piping plover nests. Wildlife Society Bulletin, 20, 143-148
- Rimmer D.W. & Deblinger R.D. (1992) Use of fencing to limit terrestrial predator movements into least tern colonies. Colonial Waterbirds, 15, 226-229
- Estelle V.B., Mabee T.J. & Farmer A.H. (1996) Effectiveness of predator exclosures for pectoral sandpiper nests in Alaska. Journal of Field Ornithology, 67, 447-452
- Mabee T.J. & Estelle V.B. (2000) Assessing the effectiveness of predator exclosures for plovers. Wilson Bulletin, 112, 14-20
- Murphy R.K., Greenwood R.J., Ivan J.S. & Smith K.A. (2003) Predator exclusion methods for managing endangered shorebirds: are two barriers better than one? Waterbirds, 26, 156-159
- Neuman K.K., Page G.W., Stenzel L.E., Warriner J.C. & Warriner J.S. (2004) Effect of mammalian predator management on snowy plover breeding success. Waterbirds, 27, 257-263
- Gulickx M.M.C. & Kemp J.B. (2007) Provision of nest cages to reduce little ringed plover Charadrius dubius nest predation at Welney, Norfolk, England. Conservation Evidence, 4, 30-32
- Gulickx M.M.C., Kemp J.B., Beecroft R.C. & Green A.C. (2007) Provision of nest cages to reduce predation of little ringed plovers Charadrius dubius at Kingfishers Bridge, Cambridgeshire, England. Conservation Evidence, 4, 49-50
- Isaksson D., Wallander J. & Larsson M. (2007) Managing predation on ground-nesting birds: the effectiveness of nest exclosures. Biological Conservation, 136, 136-142
- Pauliny A., Larsson M. & Bloqvist D. (2008) Nest Predation Management: Effects on Reproductive Success in Endangered Shorebirds. Journal of Wildlife Management, 72, 1579-1583