Seabird recovery project haunted by ghosts of cats, in the form of voracious mice
CAPE TOWN––National Geographic writer Leslie Nemo omitted two of the most ecologically significant facts about the Marion Island Special Nature Reserve from her December 14, 2018 online feature “Saving a remote island’s birds—by getting rid of its mice.”
Lauding a scheme to saturate Marion Island, 1,340 miles south of South Africa, with poisoned bait to kill mice, Nemo concluded with an appeal for readers to donate to the killing. Nemo failed to mention, however, that for 43 years the Detroit-sized island hosted feral cats, whose ancestors effectively controlled the introduced mouse population for decades.
Native mouse predators
Nemo also neglected to mention that among the 42 bird species observed at Marion Island, of whom 29 are considered resident, are at least five known or suspected mouse predators, including brown skuas, kelp gulls, Kergulen terns, and both Arctic and Antarctic terms.
All five are nest predators of other birds, potentially contributing to population declines in other species. Yet at the same time they are now the first line of defense against mice for every bird species nesting at Marion Island, and will especially be vulnerable from secondary poisoning, through eating poisoned mice.
But no nocturnal mouse predators
The threat mice pose to young and nesting birds on Marion Island is thus not so much an absence of predators as it is an absence of nocturnal predators, who hunt mice while they are most active, during the long Antarctic winter nights, coinciding with the summer months in the northern hemisphere.
When exactly mice reached Marion Island is uncertain, but at least 103 vessels visited, mostly on sealing and whaling expeditions, between 1799 and 1913. Seven of those vessels were wrecked nearby. Though few humans survived those wrecks, some mice might have floated to land with debris.
Survivors of the 1908 wreck of the Norwegian vessel Solglimt, which occurred close enough to shore for recreational divers to sometimes visit, salvaged enough materials and supplies to live on the island for several months before they were rescued. They perhaps inadvertently brought mice ashore too.
Mice “colonized every corner”
Regardless of exactly how and when mice reached Marion Island, they long since “colonized almost every corner of the island, which was declared a Special Nature Reserve in 1995,” reported Tony Carnie of the KwaZulu-Natal Mercury in 2015.
Annexing Marion Island in 1947, South Africa established a research station on the northeast shore in 1948.
Plagued by mice, the resident staff had five cats delivered from the mainland in 1949. Some of the cats and their descendants were kept as pets until 1982. Others went feral.
“By 1977,” summarizes Wikipedia, “there were approximately 3,400 cats on the island, feeding on burrowing petrels in addition to mice, and taking an estimated 455,000 petrels a year. Some species of petrels soon disappeared from Marion Island.”
Killing the cats
Recounted Vox Felina blogger Peter Wolf on May 14, 2015, “It took 19 years to exterminate approximately 2,200 cats,” by methods including “poisoning, hunting and trapping, dogs, and introducing feline distemper.”
Killing so many cats also suppressed breeding in the harsh Antarctic climate.
“In 1991,” Wolf finished, “eradication of cats from Marion Island was complete. It remains the largest island from which cats have been successfully eradicated.”
That took the brakes off the mouse population.
Mice learn to “scalp” albatross chicks
Reported University of Cape Town ornithologist Ben Dilley in the journal Antarctic Science, “The first mouse-injured wandering albatross chick was found in 2003. In 2009, the first ‘scalpings’ were detected: sooty albatross fledglings were found with raw wounds on the nape.
“In 2015, mice attacked large chicks of all three albatross species that fledge in autumn: grey-headed, sooty, and light-mantled. Filming at night confirmed that mice were responsible for the wounds.”
Also afflicted were grey and white-chinned petrels, respectively considered “near-threatened” and “vulnerable” by BirdLife South Africa.
“Right now, only a small percentage of the island’s albatross population is dying from the attacks,” wrote Nemo for National Geographic. “But ornithologists anticipate the problem will get worse,” Nemo said, because “climate change is creating warmer winters that kill off fewer mice. As a result, the island’s population has outgrown its supplies of the mice’s normal food sources, including weevils, moths, and seeds, explains Otto Whitehead, an ornithologist who earned his Ph.D. at the University of Cape Town by working on the island.
“Albatross nests, perpetually warmed by the birds’ body heat, entice mice to burrow into the insulated ground below them,” Whitehead told Nemo. “Petrels’ underground homes are even better; mice simply move in while the birds are still there.”
Mouse predation “accounted for about 10% of albatross chick deaths in 2015,” Nemo reported. “Having never dealt with these attacks in the past, the birds have no defenses.”
Under cover of darkness
Critical to realize is that while all of the Marion Island nesting bird species hatch during the months of long daylight, albatross fledglings are much slower to mature than the fledglings of the other Marion Island birds.
The immature albatross chicks are still in their nests during the months of long nights, receiving only occasional brief feeding visits from their parents. Those coldest, darkest months appear to be when the mice are hungriest and most aggressive.
Though some mouse predation on albatross and petrel chicks may have occurred for decades, it has intensified to easy visibility and apparent population significance since the cat eradication.
Plan to poison every square meter
Therefore, Nemo explained, “In 2020, BirdLife South Africa, a nonprofit conservation organization, and the South African government will team up to drop poisoned bait via helicopter over every square meter of the island. Missing bait in even one 20-meter-by-20-meter patch could allow a few mice to survive and reproduce, which would ruin the entire mission.”
BirdLife South Africa ornithologist Ross Wanless told Nemo that poison remaining after killing mice “will degrade and wash into the ocean at undetectable levels.”
But what else will the poison kill meanwhile?
Rounding up thousands of sheathbill
“Albatross are fish-eaters and so are unlikely to pick at the bait or the poisoned mice,” Nemo said. “But scavenger species like the lesser sheathbill, which will be overwintering on the island during the drop, just might. To protect them, another team of researchers will put the birds into temporary captivity. The sheathbills will be held and fed until the conservationists are sure the island is clear of fatal temptations.”
Bear in mind that the overwintering Marion Island lesser sheathbill population is believed to be in the thousands, far more than might easily be captured in rugged habitat, much of it swampy, amid daily snow, sleet, wind, and freezing rain.
How much harm may come to sheathbills in capture, holding, and through habituation? The short answer is, nobody knows.
High risk for pilots, too
“If that doesn’t sound difficult enough,” Nemo offered, “Marion Island sits in a stretch of ocean notorious for its brutal winds. Even with extra days to accommodate impossible flying conditions, the pilots will have to be incredibly skilled to pull off this mission.”
Nemo wrote less than two months after New Zealand Department of Conservation biodiversity ranger Scott Theobald, 59, fellow ranger Paul Hondelink, 63, and pilot Nick Wallis, 38, were killed when Wallis’ Hughes 500 helicopter crashed near Wanaka Airport soon after takeoff on a mission to cull Himalayan tahr.
New Zealand Department of Conservation director general Lou Sanson credited Hondelink with developing the Compound 1080 gels that the agency uses to kill brush possums, and said Hondelink had “invented” the use of “Judas goats” to lead hunters to feral goats.
Neuter/return might have done the job
Rather than risk losing pilots and rare and threatened species, there was an easier path for conservationists at Marion Island, Alley Cat Rescue founder Louise Holton pointed out when the mouse poisoning campaign was proposed in 2015.
“Remote islands are complicated,” Holton acknowledged, but “Even though taking veterinarians to such a remote region would have been challenging,” she added, neuter/return “may have been a better use of money, as the cats would have kept the mice under control. And even though they may still have caught some birds, the overall damage to birds would have been minimized.”
Who will fund the poisoning?
Unclear is whether the funding that the proposed mouse extermination project will require can actually be raised.
As of December 16, 2018, according to the Mouse Free Marion website, 143 individuals had donated only 1.3% of the anticipated project cost, with no corporate sponsors committed yet.