The Calf of Man

The Calf of Man is a 250ha island lying only 500m to the south of the Isle of Man (read more here). It was once home to thousands of nesting Manx shearwaters, the burrowing seabirds that got their common name from these islands. However, brown rats (also known locally as ‘longtails’) landed on the island from a shipwreck in the late 1700s. The rats proceeded to wipe out the huge Manx shearwater colony, and since then only a few of these iconic birds have managed to breed on the Calf. During the autumn and winter of 2012-2013, a team from Manx National Heritage, the Manx Wildlife Trust, Manx Birdlife, the Isle of Man Department of Environment, Food and Agriculture, the Animal Health and Veterinary Laboratories Agency (AHVLA) and the Royal Society for the Protection of Birds worked to eliminate rats from the Calf of Man and make the island safer for all nesting seabirds and encourage the resurgence of the shearwater colony.

The small island is dominated by maritime heath, bracken/bluebell and grassland both semi-improved and maritime. Small patches of wet flushes, marshy grassland and rock outcrops all go to form a diverse landscape. Over 340 species of plant have been recorded. Red-billed choughs breed on the sea cliffs and Atlantic puffin once bred on the island but their decline and eventual disappearance are likely to be a result of the rats’ arrival.

The rats and the seabirds

Rats will eat the eggs and chicks of almost all nesting seabirds that they can get to and have been known to kill the adult birds too. There is evidence from all over the world that where rats are present on seabird islands, the seabirds suffer large losses from rat predation. However, not all is lost. There are increasing numbers of programmes to remove rats from seabird islands and there is dramatic evidence of their success even in local waters. Following a rat eradication programme on Lundy Island in the Bristol Channel between 2002 and 2004, monitoring of burrows in 2008 showed an estimated 1,081 pairs of Manx shearwater, an increase of 250% in numbers since 2001.

The Calf project team expects that seabird breeding productivity on the island will start to recover immediately following the eradication. An anti-coagulant bait block, a “first generation anti-coagulant” called Coumatetralyl, was used to kill the rats. This acts by blocking the synthesis of vitamin K and causing internal haemorrhage. The bait was administered in boxes or pipe tunnels to which rats could gain access. Simple blocking devices were needed to discourage entry by young rabbits and birds such as crows.

Poisoned rats typically die below ground in burrows and are unlikely to be eaten by predators such as ravens and gulls. Although secondary poisoning of non-target mammals or birds could not be ruled out, the use of Coumatetralyl was particularly appropriate for environmentally sensitive applications due to its low persistence and low toxicity to birds. A risk assessment was completed for key non-target species on the island particularly for the wood mouse which, although not recorded on the Calf for some time, may still survive. Live traps were maintained in the hope of catching mice for transferral to a safe place and return following the eradication operation. However, no wood mice were found, though the project team suspect that some have survived the rodenticide deployment. Pigmy shrews, an insectivore unlikely to be attracted to the poison bait and Loaghtan sheep were not affected by the poison through careful management of the bait. To the surprise of the team, some young rabbits seemed to be attracted to the chocolate wax blocks used as non-toxic monitoring bait after the eradication phase.

Although also an invasive mammal, rabbits are desirable on the Calf as they provide burrows which shearwaters and (hopefully) puffins can use. Together with Loaghtan sheep, they help keep the grass short which benefits other wildlife such as choughs and low-growing herbaceous plants.

How do you know if eradication has worked?

Following the main eradication phase, chocolate wax-blocks were placed around the Calf, particularly to the coastal fringes where any re-infestation might occur.  It will be necessary to monitor these points into the foreseeable future and certainly for the first two years after eradication in case there are any remaining rats not dealt with during the intensive baiting phase.

Is there a risk of re-invasion?

The project team cannot be sure that rats will not re-colonise the Calf of Man, but they can minimise the risk of a new invasion. Quarantine for all boats and ships that plan to land on the island will need to be enforced and it is hoped that all boat owners who like to frequent the Calf will co-operate fully with all requirements.

Unauthorised landings and shipwrecks will remain a threat but much less so than in the past because of more hygienic conditions on board and tighter controls on landing.

Since rats are able to swim, it is conceivable that they could reach the Calf across the Sound via Kitterland but it is anticipated that the generally strong tidal conditions may reduce this risk.  Monitoring will continue on Kitterland periodically to detect any build up of rat numbers and allow a fast response to eliminate them.

At the time of writing the project team remain cautiously optimistic, despite one suspicious incidence of rat sign which has been dealt with by means of localised intensive baiting.

Do similar situations exist in the Channel Islands?

Yes. Rats may be the main reason that puffins are so rare in Jersey and shearwaters barely survive in Sark. It is probably hundreds of years since a Jersey puffin nested down a proper earth burrow, those that have nested have done so in rock crevices and fissures that the rats can’t reach. Manx shearwaters and storm petrels were probably also widespread here once. Today the Islands’ only real hope for burrowing seabirds is Burhou. This island is rat-free but if any rats ever reached it we could lose all these
puffins. There are plenty of rats on our north coast and while it would be much harder to remove them from areas like Tête des Plémont it is not necessarily impossible. But it would take hard work and a long term commitment and, who knows, maybe Jersey’s puffins could once again be safe and shearwaters once again nest here.

Sincere thanks to Kate Hawkins, Curator: Natural History, Manx National Heritage for helping compile this report.