Action: Physically protect nests with individual exclosures/barriers or provide shelters for chicks of ground nesting seabirds
- A before-and-after study from Japan found an increase in fledging rates of little terns Sterna albifrons following the provision of chick shelters and other interventions.
- Two studies from the USA and Canada found reduced predation of tern chicks following the provision of chick shelters.
- A small study from the USA found low levels of use of chick shelters, except when predators were present.
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 before-and-after study from 1978-1980 at seven least tern Sterna antillarum colonies on Nantucket Island, Massachusetts, USA (Jenks-Jay 1982) found that predation rates on chicks were greatly reduced following the provision of chick shelters (43 cm high cones made from 11 slats, with a 66 cm basal diameter), compared to previous years. In 1978, a pair of American kestrels Falco sparverius ‘greatly reduced’ tern productivity at a colony by removing a tern chick approximately every 15 minutes for two hours, whereas a pair of northern harriers Circus cyaneus reduced productivity at another colony by 80% in 1979 (four chicks from 20 nests escaping). In 1980, with shelters present, no kestrels or harriers were seen hunting within the tern colonies, although they were present in the vicinity.
A small before-and-after study in 1990 study on a breakwater in Lake Erie, Canada (Burness & Morris 1992) found that no common tern Sterna hirundo chicks were predated by herring gulls Larus argentatus or ring-billed gulls L. delawarensis over 12 days following the provision of small plywood shelters (two 12.5 x 25 cm rectangles attached to form a 10 cm high triangular shelter), compared with ten chicks being predated in the eight days between first hatching and shelter provision. A total of 29 chicks were studied, with 11 disappearing (six before shelter provision and five after) in addition to those predated.
A before-and-after study in 2001-2002 in Tokyo, Japan (Hayashi et al. 2002) found that the provision of 200 chick shelters on the roof of a sewage plant in 2002, combined with the provision of nesting substrate, appeared to increase the fledging rate of a little tern Sterna albifrons colony, compared with 2001 when birds were first observed and before habitat alterations (23% of 2,665 eggs fledged in 2002 vs. 1.5 – 2.1% of 335 eggs in 2001). The nesting substrate consisted of fine-grained (2-3 mm) ‘dried sludge’ spread over 2 ha, with 30 tonnes of shell fragments, while 38% of the rooftop was painted white. Chick shelters consisted of a sheet of wire mesh spread across two bricks.
A small study in 2003 on two warehouse roofs in Texas, USA (Butcher et al. 2007) found that least tern Sterna antillarum chicks did not use wooden shelters more than would be expected by chance. However, on 18% of occasions when adults were observed mobbing predators (n = 39), chicks were seen using either artificial plants or skylights as cover (on two occasions chicks ran towards structures, on five occasions chicks were already in cover).
- Jenks-Jay N. (1982) Chick shelters decrease avian predation in least tern colonies on Nantucket Island, Massachusetts. Journal of Field Ornithology, 53, 58-60
- Burness G.P & Morris R.D. (1992) Shelters decrease gull predation on chicks at a common tern colony. Journal of Field Ornithology, 63, 186-189
- 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
- Butcher J.A., Neill R.L. & Boylan J.T. (2007) Survival of least tern chicks hatched on gravel-covered roofs in north Texas. Waterbirds, 30, 595-601