Prickly problems for breeding waders

Spoiler alert – hedgehogs are the killers

Machair thinner (JC)

Hedgehogs – an unwelcome addition to the unique machair habitat  (Photo: John Calladine)

The chain of Hebridean islands from the northern tip of North Uist to the southern tip of South Uist are special places to visit in the summer. Nowhere else in the UK will you find higher concentrations of breeding waders. In 1983, Fuller et al. estimated that this region held a third of the UK’s breeding Dunlin and a quarter of its Ringed Plover. (Populations of breeding waders and their habitats on the crofting lands of the Outer Hebrides)

Catley Hedgepig

Photo: Graham Catley

The islands are still special but, as a consequence of the spread of hedgehogs, following the introduction of just four individuals in 1974, numbers of breeding birds have dropped dramatically. The addition of this extra predator, which can feed by day and night and possesses a coat of spines that can withstand aerial attacks by parent birds, has long been associated with the decline.  But how much of the blame lies with the hedgehog and what can be done to support wader populations?

Brief history

close up RP TGG

A quarter of the UK’s breeding Ringed Plover nest within the Outer Hebrides (Tómas Gunnarsson)

The Outer Hebrides, in north west Scotland, used to be a haven for breeding waders, many of which nest on machair, fertile coastal grasslands along the west coast.  It’s a mosaic of habitats that includes arable and pastoral farmland, pools and sand dunes. The waders that nest here have been subject to predation, from gulls, corvids, rats etc. but the hedgehog was an unwelcome addition. With no foxes or badgers to eat them, no fleas to carry diseases and a good supply of easily-accessible eggs, hedgehogs have thrived and spread.

Work in 1998, led by Digger Jackson, showed that the hatching success for Dunlin, Lapwing, Redshank and Snipe clutches was 2.4 time higher in areas from which hedgehogs had been excluded. This was published in the Journal of Applied Ecology in 2000.

Breeding waders were surveyed in 1983, when hedgehogs were restricted in their distribution to small parts of South Uist. By the time of the next extensive survey in 2000, hedgehogs were well-established across South Uist and Benbecula, and the first individuals had reached the southern end of North Uist Uist. In just 17 years, numbers of breeding waders declined by 39% in the areas with hedgehogs, while there was an increase of 9% in North Uist. There were differences for all wader species but particularly for Redshank and Lapwing. The results were published in Biological Conservation in 2004. A subsequent control programme, to assess whether eradication might be a practical option, was expected to arrest the decline in wader numbers and promote recovery. Sadly, population levels have not recovered as well as expected which begs the question – ‘are hedgehogs the only problem?’

Further large-scale surveys were carried out in 2007 and 2014

Walking transect cropped (JC)

Rob Fuller, walking a transect (John Calladine)

The 2007 survey is written up in Bird Study. (Changes in the breeding wader populations of the machair of the Western Isles. Scotland, between 2000 and 2007).

The 2014 survey is written up in Scottish Birds (Calladine, J., Humphreys, E.M. & Boyle, J. (2015). Changes in breeding wader populations of the Uist machair between 1983 and 2014. Scottish Birds 35: 207-215).

In the period between 1983 and 2014, the greatest overall proportional declines were of Dunlin and Ringed Plover (72% and 70% respectively). Snipe declined overall by 45% and Lapwing by 14% while, in contrast, Redshank increased by 8% and Oystercatcher by 74%. The changes were not the same in the zones with and without hedgehogs. The deviation in trends was most marked for Redshank (which increased by 91% in the northern zone, with fewer hedgehogs, while decreasing by 22% in the southern zone) and Snipe (much bigger decreases in the south). Dunlin and Ringed Plover declined markedly in both zones, though less so in the northern zone. Any difference in trends was less apparent for Lapwing, although it did appear that they may have fared less well in the southern zone.

Are hedgehogs the main problem?

Predation event 1 (SNH video grab)

A hedgehog ignores nest defence efforts of pair of Lapwing (screen-grab from SNH surveillance camera)

A recent report, prepared for Scottish Natural Heritage by a consortium comprising the British Trust for Ornithology, the James Hutton Institute and MacArthur Green Ltd., aims to quantify the importance of predation by hedgehogs, relative to that by other predators.  The report includes a summary of the changes in numbers for the various wader species during a forty-year period, based on occasional broad-scale surveys and more regular monitoring of specific study sites.  The focus, however, is on results from studies using nest cameras, temperature loggers and fixed point observations in the 2012, 2013 and 2014 breeding seasons, measures of disturbance, transect work to assess productivity, and some land use and habitat recording.

report coverCalladine, J., Gilbert, L., Humphreys, E.M., Fuller, R.J., Robinson, R.A., Littlewood, N.A., Mitchell, R.J., Pakeman, R.J. & Furness, R.W. 2015. Predation studies on breeding waders of the Uist machair: Final report covering fieldwork undertaken in 2012-14. Scottish Natural Heritage Commissioned Report No. 811. 

Hog on machair (JC)

A hedgehog in the half-light (John Calladine)

 

The principal aim of the study was to quantify the importance of predation by hedgehogs, relative to that by other predators, work that could support arguments in favour of continued and enhanced removal of these introduced predators. Cost-effective solutions may require lethal control, instead of or in addition to translocation. These are tricky decisions, especially when considered alongside the rapid decline in numbers of hedgehogs across much of the UK.

Main findings

survival table

Clutch survival rates

The latest study provides further evidence that predation by hedgehogs is having an ongoing impact on breeding success of wader nest on the Uist machair, at least some of which is additional to that of gulls, which take both eggs and chicks.

In North Uist (low density of hedgehogs) clutch survival rates were higher than on South Uist (high density of hedgehogs) for the four main study species – see table. Overall, in North Uist, 74% of

predation table

Confirmed and probable causes of predation

wader nests, where outcome was confirmed, successfully hatched chicks.  In South Uist only 45% of nests successfully hatched chicks.  Most observed losses in both areas were due to predators.

Hedgehogs were the most numerous nest predators identified using nest cameras and other evidence, accounting for over half of occurrences, with all confirmed instances of predation by hedgehogs occurring in the ‘high hedgehog density’ areas. In these high density areas, hedgehogs were responsible for 35% of nest failures. Although summer nights are shorter than in much of the UK, 45% of clutch incubations ended during darkness in high hedgehog density areas, compared to 29% in low density areas, again suggesting that mammals are the main predators, with hedgehogs the most likely culprits.

Predation event 2 (SNH video grab)

A Common Gull takes an egg – and may be back for more (screen-grab from SNH surveillance camera)

Although avian predators (gulls, raptors and corvids, which are diurnal predators) were recorded most frequently in the high hedgehog density areas, observers did not find a difference in the frequency of predation attempts by avian predators between these areas and areas with fewer hedgehogs. The number of successful attacks by avian predators witnessed during fixed point observations was small (24 in 460 hours of observations spread over three years) and more successful avian predation events were recorded in low hedgehog density areas (n = 15) than in high hedgehog density areas (n = 9), despite lower abundance of avian predators in the low hedgehog density areas. There is a suggestion that avian predation may, to some extent, replace hedgehog predation in areas with low hedgehog density, especially at the chick stage, but this interpretation is based on a small sample size.

Other potential causes of wader declines may be associated with land use changes within the machair. These are discussed in a 2014 Bird Study paper by Calladine et al.

Conclusions

hog-predated nest (David MacLennan)

Evidence of hedgehog predation (David MacLennan)

It is pretty clear that hedgehogs are causing significant problems for nationally and internationally important populations of breeding waders, two of which are red listed species of conservation concern (Lapwing and Ringed Plover) and the rest of which are amber-listed.  It is also clear that efforts to control hedgehogs have had limited success.

Eradication is a contentious issue. Hedgehogs may have been introduced but, having become established on the Uists, the species has attracted significant support from a number of animal protection organisations, members of which don’t want lethal control methods to be employed. Translocation – the current control method – is a costly alternative.

SNH has demonstrated a political will to try to tackle the hedgehog issue and has applied to the EU for funding for an eradication programme. There’s more about the Uist Wader Project  on the Scottish Natural Heritage website.

Given current financial constraints, EU funding seems to be the most likely way to deal with this man-made predation issue. Any eradication programme may rely on proving that translocated hedgehogs survive. If they don’t, then there is probably a choice between using lethal control or further losses of red-listed and amber-listed wader species. At that point, is there going to be the political resolve to embark on a plan to remove all of the hedgehogs? Wader biologists hope that funding can be found to tackle this prickly issue.


 GFA in Iceland

WaderTales blogs are written by Graham Appleton, to celebrate waders and wader research.  Many of the articles are based on previously published papers, with the aim of making wader science available to a broader audience.

@grahamfappleton

Tracking waders on the Severn

Birdwatchers are being asked to help with some cutting-edge science, simply by reporting sightings of colour-dyed Dunlin and colour-ringed Curlew and Redshank.

Severn 1 WWT (2)

The Severn: an empty estuary or a food-rich haven? (Corinna Blake)

The tides that create unique feeding opportunities for waders and other waterbirds on the Severn can potentially be harnessed to produce large amounts of clean energy. New impact assessment work aims to see how a development that would bring big benefits to the local economy might be carried out with as little negative environmental side-effects as possible.

Colour ringed Curlew by Kane Brides

Birdwatchers are asked to look out for colour-ringed Curlew and Redshank (Kane Brides)

The Severn is a great place for birds, especially waders, attracted to the area by mud that has high densities of mud-loving invertebrates such as ragworms. It is designated as an SPA, because of its importance to wintering species such as Bewick’s Swan, Curlew, Dunlin, Pintail, Redshank and Shelduck, and the spring passage of Ringed Plovers. There’s more about the SPA on the JNCC’s website.

The British Trust for Ornithology and Wildfowl & Wetlands Trust scientists have been awarded a contract to undertake an Environmental Impact Assessment for the proposed Cardiff Tidal Lagoon  by Tidal Lagoon Power, with BTO focusing on waders and WWT on ducks. It’s a unique opportunity not only to inform conservation planning but also to answer questions about the winter ecology of some key species. The team aims to:

  • Validate and refine methods developed by Richard Stillman (Bournemouth University) that predict bird distributions from food availability. (It’s easier to map food distribution than to follow bird movements)
  • Understand patterns of movement of species that use the proposed development area and other key sites, such as the Gwent Levels.
  • Track birds in order to identify feeding and roosting areas that are used when birds are hard to observe – in poor weather, at night and at all stages of tide.
  • Work out how mobile birds are, in order to propose ways in which lost feeding opportunities might be replicated as close by as possible.

Tagging

Tag on Curlew back (head covered to keep bird calm) by Lucy Wright

Tags like this should reveal how waders and ducks use the Severn estuary. Curlew by Lucy Wright.

Tagging is an important part of the project. Four-gramme Pathtrack tags are being glued to the backs of a sample of Curlew, Redshank, Shelduck and (hopefully) Shoveler. These should stay on for a couple of months, during which time movements will be logged every 90 minutes and downloaded using UHF receiving stations set up around the Severn. These four species have been chosen because, for each, more than 10% of the estuary’s population lies within the proposed footprint of the tidal lagoon.

Colour-rings

Colour ringed Redshank by Emily Scragg

This colour-ringed Redshank may well breed in Iceland but which areas of mud-flat does it use in the winter? (Emily Scragg)

Tagged Redshank and Curlew are also being colour-ringed, alongside others that are not being tagged. By collecting reports of colour-ringed birds from birdwatchers, the BTO team will be able to monitor the efficacy of the tag down-load process and keep a track of movements when the tags stop transmitting. As an added bonus, the colour-rings may generate some new information about the breeding sites of waders that winter on the Severn.

Sightings of colour-ringed birds would be very much appreciated. Five rings have been used on both Redshank and Curlew. Please submit sightings (date time and ideally a six-figure grid reference) to Emily.scragg@bto.org who would also be interested in “ratio counts” of flocks of birds – simply the number of colour-rings and the size of the flock.

Colour-dyed Dunlin

The Severn Estuary holds an estimated 3.2% of the European wintering population of the alpina race of Dunlin, birds that breed from Siberia across to northern Scandinavia. Dunlin are too small to carry transmitters that can be used with base stations so the team has gone back to traditional picric dye in order to look at the mobility of flocks. Any sightings of colour-marked Dunlin will be appreciated by emily.scragg@bto.org. Where possible, please submit ratio counts broken up into yellow/orange on breast (adults), yellow/orange on the rump (juveniles) and unmarked birds, together with date, time and location (ideally with six-figure reference).

Colour-dyed Shelduck

shelduck Kane Brides

The neck of this tagged Shelduck will be dyed yellow before release (Kane Brides)

It’s not just waders.  Over 3,000 Shelduck winter on the Severn, which is more than 1% of the European population. A sample has been caught by WWT. Ringed birds have a yellow/orange dye mark on the normally white plumage on the neck/upper breast (between the dark green head and the brown breast band). No Shoveler have been caught yet but the aim will be to put a similar dye-mark on these birds too.  Sightings of dye-marked ducks should be reported to Ed.burrell@wwt.org.uk

Impact Assessment

The consortium of organisations that is working on this new tidal-power study is well placed to combine impact assessment with high quality wader science. By focusing on Curlew and Redshank, both red-listed species of conservation concerned, it is to be hoped that more will be learnt about the winter feeding ecology of these two species. The BTO team has already discovered that Redshank fly further at night than was previously thought and hope to get to understand some of the pressures facing Curlew, now classified as globally near-threatened (see separate WaderTales blog)

Severn 4 WWT

The Severn – Gareth Bradbury/WWT

This is not the first impact assessment work that has managed to incorporate research that increases the scientific understanding of wader behaviour and ecology. Two other examples are given below:

The Wash: Back in the 1970s, a plan to build huge reservoirs on the mud flats of the Wash, in which to store fresh water that might meet the growing demands of southeast England, led to a doubling of wader catching activity, intensive studies of their feeding ecology and complementary work on other taxa. Much was learned about the mobility of species within this huge estuary and the turn-over of birds within the annual cycle. A draft copy of the report is available on line at the NERC website 

Cardiff Bay: In the period 1989 to 2003, long-term studies took place to try to understand the impacts of closing Cardiff Bay and hence reducing the amount of tidal feeding area for waders. Following the development, it was shown that Redshank that had been displaced from the Bay were in poorer condition and had lower survival rates in subsequent winters.  There’s a summary on the JNCC website. The papers listed at the bottom of this JNCC web-page are essential reading for anyone trying to counter the ‘birds will simply go elsewhere’ arguments, which are sometimes put forward in favour of development work.


 GFA in Iceland

WaderTales blogs are written by Graham Appleton, to celebrate waders and wader research.  Many of the articles are based on previously published papers, with the aim of making wader science available to a broader audience.

@grahamfappleton