Action: Provide artificial nesting sites for ground and tree-nesting seabirds
- Three studies from the UK and the Azores found increases in gull and tern populations following the provision of rafts/islands or providing nest boxes alongside other interventions.
- A controlled, replicated study from the USA found that terns had higher nesting success on nesting rafts in one of two years monitored and a before-and-after study from Japan found that nesting success increased after the provision of nesting substrate.
- Five studies from Canada and Europe found that terns used re-profiled or artificial islands or nesting rafts, but pelicans did not.
- A small study from Hawaii found that red-footed boobies Sula sula preferentially nested in an artificial ‘tree-style’ nesting structure, compared to other designs.
Many seabirds require open areas of sparse vegetation to nest. These can be created either through the provision on artificial rafts and islands, described below, or through the clearing of vegetation on existing sites, which is discussed in ‘Natural system modifications’.
Supporting evidence from individual studies
A small replicated, controlled study from May-August in 1982 on a concrete breakwater in Port Colborne, Canada (Richards & Morris 1984), found that common terns Sterna hirundo nested at higher densities on two plots enhanced with clumps of mossy stonecrop and driftwood added (62% of 166 clutches in these plots), compared to plots layered with gravel (29% of clutches) or control plots of bare concrete (9% of clutches). Enhanced plots were also colonised earlier. Average clutch size and hatching rates were similar between plots (2.4-2.5 eggs/clutch and 76-86% hatching success), but the average number of chicks fledged per pair was significantly higher in enhanced (1.6) and control (1.3) plots than in gravel-layered plots (0.6). The breakwater was divided into six 5 ? 7 m plots, with two plots for each treatment.
A replicated trial in 1990 at Lake Ontario, Canada (Dunlop et al. 1991), found that common terns Sterna hirundo successfully nested on four floating wooden rafts the same season that they were installed, with at least 170 fledglings being produced (average of 1.3 fledglings/nest). Terns successfully defended the rafts from Canada geese Branta canadensis and ring-billed gulls Larus delawarensis and used all four rafts. Rafts were 5 x 5 m, covered with sand and gravel and each had six decoy terns on (see ‘Attract birds to safe areas using decoys’ for more studies on decoys).
A 1992 review of the use of artificial islands and floating platforms in 17 wetland nature reserves across the UK (Burgess & Hirons 1992) found that all seven species of gull and tern investigated used sparsely-vegetated islands and platforms at southern, coastal sites, but that nesting sites elsewhere were not used by four of the species. Sandwich terns S. sandvicensis used vegetated nesting sites at southern coastal sites, whilst black-headed gulls L. ridibundus and common terns S. hirundo nested at all sites. At one site in Kent, the provision of 20 shingle islands has attracted 350 pairs of Sandwich and common terns and 1,000 pairs of black-headed gulls. The review also examines island and platform use by grebes, divers, rails, waders and wildfowl.
A small trial in 1993-5 in western Lake Ontario, Canada (Lampman et al. 1996), found that the number of Caspian terns Sterna caspia nesting on an artificial raft increased from one pair in 1993 (raising two chicks) to 50 pairs (raising 97 chicks) in 1995. In 1995 the raft produced the majority of young in the area, due to heavy predation on mainland nests by red foxes Vulpes vulpes. The raft was 3.6 x 9.8 m, covered in sand and gravel, was anchored adjacent to a mainland subcolony and covered with a tarpaulin between April and May to discourage ring-billed gulls Larus delawarensis from nesting. Eight tern decoys were also placed on the raft (discussed in ‘Use decoys to attract birds to safe areas’), a sound system played vocalisations from a Caspian tern colony in 1993 (see ‘Use vocalisations to attract birds to safe areas’) and in 1995, eight chick shelters were added to the raft.
A replicated study in 1987-1990 of a managed wetland in Macedonia, Greece (Pyrovetsi 1997) found that the target species, Dalmation pelicans Pelecanus crispus, did not benefit consistently from artificial habitats although other waterbirds did. Two constructed rafts and one artificial island were used extensively by a variety of waterbirds as resting and foraging sites. Common terns Sterna hirundo colonised the rafts in both years (average 12 nests and 14 fledglings / raft). Dalmatian pelicans did not colonise the rafts. Many waterbirds, including pelicans, were observed roosting on the island but no successful breeding took place in 1988-1989. In April 1990, 26 pelicans colonised the islands. Thirteen nests contained 1-2 eggs each. By June, however, the pelicans had deserted the island, no eggs remained and some nests had been destroyed. The authors speculate that fisherman landed on the island and removed the eggs. Pelicans did not return to the island.
A small study at a shrubland site on O’ahu, Hawaii, USA (Rauzon & Drigot 1999), found that red-footed boobies Sula sula preferentially nested in a ‘tree-style’ artificial nest platform, compared to a transported tree, a ‘tripod-style’ platform or a linear platform. A total of approximately 15 young were produced between 1992 and 1998 from nests on the tree-style platform, which consisted of a 6 m tall beam with cross beams, providing nine potential nest sites. A transported kiawe Prosopis pallida tree and kiawe branches were used for perching and several booby nests, five tripod platforms (each providing seven nesting sites) were erected in 1992, but only three nests were built over three breeding seasons and a single linear platform providing 50 nest sites was used only once.
A before-and-after study in 2001-2 in Tokyo, Japan (Hayashi et al. 2002) found that the fledging rates in a little tern Sterna albifrons colony was higher following the provision of nesting substrate and chick shelters. This study is discussed in ‘Physically protect nests with individual exclosures/barriers or provide shelters for chicks’.
A before-and-after study at a former gravel pit in Kent, England (Akers & Allcorn 2006), found that one pair of common terns Sterna hirundo, five pairs of black-headed gulls Larus ridibundus and approximately 100 pairs of herring gulls L. argentatus nested on a series of gravel islands after they were re-profiled and lowered in February 2005 to encourage winter flooding. Vegetation was also removed from the islands (see ‘Manually remove vegetation from wetlands’). The number of birds nesting on the islands was originally high but declined until none nested there in 2002.
A controlled, replicated trial in 2003-4 at a wetland site in Wisconsin, USA (Shealer et al. 2006), found that black terns Chlidonias niger occupied 63-66% of 41 floating nest platforms provided each year (34-35% of the local population used them). Platform nests had significantly higher hatching success and nest survival rates in 2004, but not 2003. Eggs laid on platforms were significantly larger than those on natural substrates, suggesting that platforms were occupied by high-quality birds (and were therefore preferred). Platforms were 46 x 46 cm polystyrene and plywood squares, covered in hardware cloth and anchored to the lake bottom with a metal pipe (allowing vertical movement). Platforms were spaced 10–15 m apart in clusters of 5–10 and were positioned in the same location during both years of the study.
A before-and-after trial in northeast England (Morrison & Gurney 2007) found that the number of roseate terns Sterna dougallii at an island site increased following the creation in 2000 of an artificial nesting terrace and the provision of additional nest boxes (94 pairs in 2006 vs. an average of approximately 28 pairs in 1975–1999). Since 2003, all breeding pairs have used nest boxes. Before 2000 there were up to 12 nest boxes on the island, but 25 were installed in 2000 and more added each year until 200 boxes were present in 2006. Boxes were 15 x 30 x 45 cm with a 15 cm doorway; the terrace was 25 m long originally (it was extended in 2001), with three tiers, each protected by flagstones to prevent burrowing birds undermining its structure.
A before-and-after study on Praia Islet (12 ha), off Graciosa, Azores, Portugal (Bried et al. 2009), found that the breeding populations of common terns Sterna hiundo and roseate terns S. dougallii increased dramatically (from no pairs to over 1,000 and 400 pairs respectively) following the installation of nest boxes in 1996, combined with the eradication of rabbits (see ‘Control or remove habitat-altering mammals’) and habitat restoration (‘Shrubland restoration’). Fifty wooden boxes were installed in 1996 in the area with the least vegetation and the proportion of the common terns nesting in the boxes increased between 1996 and 2006. The effect of nest boxes for burrow-nesting seabirds is also discussed.
- Richards M.H. & Morris R.D. (1984) An experimental study of nest site selection in common terns. Journal of Field Ornithology, 55, 457-466
- Dunlop C.L., Blokpoel H. & Jarvie S. (1991) Nesting rafts as a management tool for a declining common tern (Sterna hirundo) colony. Colonial Waterbirds, 14, 116-120
- Burgess N.D. & Hirons G.J.M. (1992) Creation and management of artificial nesting sites for wetland birds. Journal of Environmental Management, 34, 285-295
- Lampman K., Taylor M. & Blokpoel H. (1996) Caspian terns (Sterna caspia) breed successfully on a nesting raft. Colonial Waterbirds, 19, 135-138
- Pyrovetsi M. (1997) Integrated Management to Create New Breeding Habitat for Dalmatian Pelicans (Pelecanus crispus) in Greece. Environmental Management, 21, 657-667
- Rauzon M. & Drigot D. (1999) Red-footed booby use of artificial nesting platforms. Waterbirds, 22, 474-477
- Hayashi E., Hayakawa M., Satou T. & Masuda N. (2002) Attraction of little terns to artificial roof-top breeding sites and their breeding success. Strix, 23, 143-148
- Akers P. & Allcorn R.I. (2006) Re-profiling of islands in a gravel pit to improve nesting conditions for terns Sterna and small gulls Larus at Dungeness RSPB reserve, Kent, England. Conservation Evidence, 3, 96-98
- Shealer D., Buzzell J. & Heiar J. (2006) Effect of floating nest platforms on the breeding performance of black terns. Journal of Field Ornithology, 77, 184-194
- Morrison P. & Gurney M. (2007) Nest boxes for roseate terns Sterna dougallii on Coquet Island RSPB reserve, Northumberland, England. Conservation Evidence, 4, 1-3
- Bried J., Magalhaes M.C., Bolton M., Neves V.C., Bell E., Pereira J.C., Aguiar L., Monteiro L.R. & Santos R.S. (2009) Seabird habitat restoration on Praia Islet, Azores Archipelago. Ecological Restoration, 27, 27-36