conservation > mice massacre Help for Gough Island’s birds GOUGh Island’s birds are under siege. Introduced house mice eat around one million eggs and chicks each year, threatening the existence of several bird species found nowhere else. But the good news is that plans are being developed to eradicate the mice in 2019. If all goes well, Gough will retain its status as the world’s most important seabird island. Ben Dilley and Delia Davies report their experiences on the front lines. > NOVEMBER/DECEMBER 2015 SEABIRD CONSERVATION 43 GOUGH ISLAND is spectacular: musculus, they triggered an eco­ 65 square kilometres of rugged logical disaster that only now can south africa volcanic mountains and precip­ we attempt to rectify. itous valleys rising sheer from the Cape Town sea. It lies on the edge of the Roar­ irds that live on oceanic ing Forties in the South Atlantic, islands have evolved in Tristan da Cunha midway between South Africa and the absence of land­based Gough Island Argentina, and is administered by predators,B because most terrest rial Tristan da Cunha, a UK Overseas animals were unable to colon ise Territory. Often regarded as the these remote specks of land. When SOUTH ATLANTIC OCEAN Marion Island world’s most important seabird humans reached these islands, breeding island, Gough supports they often introduced predators literally millions of seabirds of 23 such as cats, rats and even snakes, species, three of which breed al­ with cata strophic impacts. Unable in 2004 by Ross Wanless and An­ most exclusively on Gough: the to appreciate the danger posed by drea Angel, who also filmed mice Tristan Albatross, Atlantic Pet­ these strange new arrivals, island killing chicks of Atlantic Petrels rel and Macgillivray’s Prion (see birds were easy prey. As a result, and Great Shearwaters. In 2013‒14 GAfrican Birdlife, May/June 2014, more than 90 per cent of bird ex­ we spent a year on this incredible p. 10). It is also home to two en­ tinctions in the past 500 years have World Heritage Site and returned demic landbirds: the Gough Bunt­ been of island species. with horrifying new evidence of ing and Gough Moorhen. Until relatively recently, house just how pervasive the level of below A female At 380 kilometres south­south­ mice were not considered a ser­ mouse predation is. Tristan Albatross east of Tristan, Gough is seldom ious threat to seabirds; there were We wanted to assess how the sits next to her chick visited by Tristan islanders. Ac­ just a few records of mice nibbling mice’s predatory behaviour af­ which is being eaten cess is limited by the rough seas the eggs and chicks of storm pet­ fects the 16 burrow­nesting petrel by mice. Having and steep, boulder beaches. Its ex­ rels at California’s Farallon Islands species that breed on the island. evolved on oceanic is- posed western coastline thwarted and Blue Petrels on Marion Island. To do so we installed infrared lands in the absence even the hardiest of adventurers It was only in 2001 that Richard cameras in the underground nest of land-based preda- who hunted seals throughout the Cuthbert and Erica Sommer first chambers of as many petrels as we tors, these birds are Southern Ocean in the late 18th suspected that mice were having could find. This was technically unable to appreciate and early 19th centuries. Gough a significant impact on Gough Is­ challenging, because most CCTV the danger posed by was virtually the only island land’s birds. Fatal attacks on Tristan systems can’t withstand the wet these strange new where substantial numbers of Albatross chicks were confirmed and muddy island conditions. > arrivals. sub­Antarctic fur seals survived the sealing era, and the island still supports the world’s largest population of this species. How­ ever, the sealers left their mark on Gough Island: by accidentally introducing house mice Mus left Gough Island is often regarded as the world’s most important seabird island. Its densely vegetated terrain is home to thousands of surface-nesters like this Atlantic Yellow-nosed Alba- tross, but most of the breeding birds are burrow-nesting petrels that only emerge at night. previous page When this Common Diving Petrel chick hatched it was the size of a golf ball. Here it is 13 days old and a bit bigger, and although still no match for a hungry mouse, this chick did survive to fledge. 44 AFRICAN BIRDLIFE NOVEMBER/DECEMBER 2015 SEABIRD CONSERVATION 45 both eggs and chicks were targeted. we were able to confirm that Burrow­nesting petrels frequently mice were not only wounding the leave their eggs unattended for a chicks but killing them outright. few hours, which is not a problem It seems incredible that a 50­gram as the eggs can tolerate short­term mouse can kill an albatross chick chilling. However, it does create weighing more than a hundred an opportunity for hungry mice, times its own weight, but the which soon gnaw their way into camera doesn’t lie. the eggs. Attacks start when a single Our observations confirmed mouse gnaws at a spot on the that house mice are significant chick’s body, commonly its rump predators of petrel eggs and or back. The chick would occa­ chicks on Gough Island and that sionally try to shake the mouse species are impacted irrespective loose, but the rodent would keep of when they breed (but winter returning until it had created a breeders definitely fare worse, at small, open wound. Once the flesh least among the larger species). is exposed, other mice join in, fur­ Mice are effective predators, kill­ ther enlarging the wound. After a ing chicks within hours of hatch­ few nights the chick dies, and the ing while still brooded by their carcass is usually scavenged by parents, and also tackling large skuas or giant petrels. Our cam­ chicks many times their body eras revealed that more than 90 he good thing about is­ Diomedea dabbenena chicks at Eradicating mice from MORE THAN 90 PER size. However, small species per cent of chicks were killed by lands is that eradication Gough Island, South Atlantic Ocean.’ an island as large as CENT OF BIRD probably are at greater risk. We mice. In 2014, fewer than 10 per is a viable option – pro­ Avian Conservation and Ecology, Gough requires the couldn’t find any nests of the four cent of Tristan Albatross breeding Tvided adequate measures are put 10, 10.5751/ACE-00738-100105. use of helicopters to EXTINCTIONS IN storm petrel species that breed attempts were successful. In the in place to prevent subsequent Dilley, B.J., Davies, D., Bond, disperse rodenticide bait THE PAST 500 on Gough and, based on trends absence of introduced predators, reintroductions. Scientists from A.L. and Ryan, P.G. 2015. ‘Effects over the entire island. YEARS HAVE BEEN in the numbers coming ashore these birds usually rear about 70 the UK BirdLife partner, the Roy­ of mouse predation on burrow- OF ISLAND SPECIES at night, populations of Grey­ per cent of their chicks. al Society for the Protection of ing petrel chicks at Gough Island.’ backed, White­bellied and Black­ Birds (RSPB), and from the Percy Antarctic Science, DOI: 10.1017/ relatively infrequent. The only bellied storm petrels have been oss Wanless suggested FitzPatrick Institute at the Uni­ S0954102015000279. Common Diving Petrel chick dis­ particularly hard hit. that mice are particularly versity of Cape Town have been covered wasn’t attacked, despite problematic for birds studying the feasibility of eradi­ mice regularly visiting the nest ough is also home to whenR they are the only introduced cating mice by using helicopters chamber and hassling the tiny three species of albatross. predator. Under these conditions, to spread rodenticide bait over MEGA MICE chick which, at least initially, was Currently, mice have lit­ mice attain very high densities the entire island. Such initiatives The size factor much smaller than the mice. How­ Gtle impact on the two summer­ (close to 300 mice per hectare) have been successful on other ats are well-known predators of seabird chicks ever, by midsummer the mice were breeding species, killing only in summer. As winter sets in, sub­Antarctic islands. rbut, prior to the observations on Gough island, above Macgillivray’s We ended up building our own starting to take their toll. All the the occasional chicks of Sooty food resources for mice dwindle, In June 2015 the RSPB an­ mice were thought to be too small to pose a serious Prions were recently system, using cameras fitted with Macgillivray’s Prion chicks in Pri­ and Atlantic Yellow­nosed alba­ forcing them to seek alternative nounced phase 1 of the Gough Is­ threat to seabirds. discovered breeding 120˚ wide­angle lenses to film the on Cave were killed within weeks trosses. However, the much larger sources. Unfortunately, seabird land eradication attempt. If the Gough’s mice are unusual in that they are 50 per on Gough Island. Even tight interior of a burrow and illu­ of hatching. As winter approached Tristan Albatrosses rear their chicks are an easy target. Burrow­ £7.6­million funding required cent larger than other island populations of house though the birds breed minated by 12 infrared LEDs dif­ and alternative food supplies for single chick through winter, and nesting petrels and prions leave can be secured and all complex mice, which might give them an edge when it in midsummer, the fused with kitchen wax wrap and the mice dwind led, the carnage they are particularly prone to at­ their chicks alone a few days af­ logistical arrangements go ac­ comes to tackling seabird prey.