Does light grazing of saltmarshes cause nest mortality in Common Redshank?
LINKED PAPER
Light grazing of saltmarshes is a direct and indirect cause of nest failure in Common Redshank Tringa totanus.
Elwyn Sharps, Jennifer Smart, Martin W. Skov, Angus Garbutt & Jan G. Hiddink. 2015.
IBIS. DOI: 10.1111/ibi.12249
The Common Redshank Tringa totanus is known as the “the guardian of the marsh” because they make a loud warning cry at the first sign of an intruder, and are often seen on top of fence posts, looking down over their chicks.
Throughout Europe the populations of many breeding wader species have decreased, with a number of these declines linked to agricultural intensification and the associated loss of breeding habitats. Saltmarshes are internationally important for Redshank, in Britain they hold around half of the breeding population. This is around 9% of north-west Europe’s estimated 100,000 – 172,000 breeding pairs. However, recent surveys of British saltmarshes found a 53% decline in nesting Redshank pairs between 1985 and 2011 and it has been suggested that conservation management has failed to reverse historic Redshank declines. Grazing is perhaps the most widespread conservation management technique on saltmarshes, so this project investigated the effects of grazing on Redshank nest survival.
Grazing by domestic livestock can change the suitability of the habitat for Redshank by limiting or creating availability of vegetation patches, which the birds use for nesting. More intensive grazing leads to a very short, uniform sward while lighter grazing results in a more uneven patchy sward with diverse heights. Redshank population declines on British saltmarshes have been linked to changes in grazing management as breeding densities are higher in light and moderate grazing than on heavily grazed or un-grazed saltmarshes.
Livestock can also reduce Redshank nest survival by trampling nests. Nest predation rates may also be indirectly increased by grazing, either through decreasing nest cover and vegetation heterogeneity, therefore making it easier for predators to find nests, or by reducing the amount of unoccupied patches of long vegetation that must be searched by a predator in order to find a nest. An analysis of European studies found that >50% of wader nests were predated in >55% of the 544 examples that they reviewed. Potential Redshank nest predators on British saltmarshes include corvids Corvus spp., gulls Larus spp., Red Foxes Vulpes vulpes, Stoats Mustela ermine and non-native American Mink Neovison vison.
To balance this potential conflict between the creation of a suitable vegetation sward whilst minimising grazing-induced nest mortality, densities of ~1 cattle per hectare have been recommended. This falls within the UK Environment Agency definition of light saltmarsh cattle grazing of 0.7-1 young cattle per hectare, present from April to October. But recent evidence suggests that these light grazing levels may be too high for breeding waders and one study found that approximately 80% of experimental false wader nests were lost to trampling and predation in a Baltic coastal meadow with 0.83 cattle per hectare. On saltmarshes the effect may be even stronger, as Redshank tend to nest in vegetation communities associated with the high-mid marsh, which is typically closer to the landward side of the saltmarsh, and it has been suggested that grazing pressure can be higher in these areas.
In our IBIS paper, we estimate Redshank nest survival over a range of livestock densities on six lightly grazed saltmarshes in the Ribble estuary in north-west England. We hypothesise that higher grazing pressure results in increased nest mortality: (1) directly through cattle trampling, and (2) indirectly through grazer modification of habitat that accelerates predation risks.
Results showed that risk of nest loss to trampling increased from 16% at 0.15 cattle per hectare to 98% at 0.82 cattle per hectare. The risk of a nest being predated increased from 28% with no grazing to 95% at 0.55 cattle per hectare based on all year grazing data. These results suggest that even light conservation grazing at less than one cattle per hectare can reduce Redshank nest survival rates to near zero.
This work does not suggest that stopping livestock grazing on saltmarshes altogether will result in increased breeding populations of Redshank. Cessation of grazing in previously grazed saltmarshes can result in reductions in breeding Redshank as the vegetation becomes dominated by tall uniform vegetation. However, it may be appropriate to reduce saltmarsh grazing intensities, or change the timing of saltmarsh grazing to reduce the number of livestock present during the Redshank breeding season.
Further reading
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Beintema, A.J. & Muskens, G.J.D.M. (1987) Nesting Success of Birds Breeding in Dutch Agricultural Grasslands. Journal of Applied Ecology 24: 743-758.
Brindley, E., Norris, K., Cook, T., Babbs, S., Brown, C.F., Massey, P., Thompson, R. & Yaxley, R. (1998) The abundance and conservation status of redshank Tringa totanus nesting on saltmarshes in Great Britain. Biological Conservation 86: 289-297.
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IMAGE CREDITS:
Common Redshank © Klaas Felix Jachmann
Cattle © Adam Cross
Redshank nest; trampled eggs © Christine Tansey
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