The Rodent Situation (Not Your Grandfather’s Mouse)

Spawned from undersea volcanoes and thousands of miles from any continent, the Hawaiian Islands harbor very few indigenous land animals. Once in a while a bird or insect made its way here – perhaps blown in by a storm – and over the eons a few survived and were able survive. From these the entire Hawaiian terrestrial fauna evolved. Most mammals don’t do overseas travel very well, as evidenced by the fact that the only native mammal in the entire Hawaiian archipelago is the reclusive Hawaiian Hoary Bat (‘ope‘ape‘a). After human technology developed to the point where people could travel long distances by boat, the situation changed.  Some of the earliest Polynesian explorers that made their way the shores of these islands in seagoing canoes unwittingly carried with them the Polynesian rat (Rattus exulans). When Europeans began to visit the islands they brought along their own rat:  black rat (Rattus rattus; sometimes called the “ship rat”). Also onboard were mice (Mus musculus).

Non-native rodents often found on oceanic islands. Top: House mouse (Mus musculus). Photo courtesy of JN Stuart (Flickr / Non-commercial Creative commons license). Bottom: Black rat (Rattus rattus). Photo courtesy Hermann Falkner (Flickr / Non-commercial Creative commons license)

Rats and mice have followed humans around for millennia, inhabiting our homes, barns, fields, roaming the streets and back alleys of cities and towns. They are well known thieves of food and have a catholic diet. Sure, they like cheese and peanut butter, but contrary to Walt Disney, they are not vegetarians and are known to a variety of animal prey and will even resort to cannibalism when the situation demands. People have battled with rats and mice for centuries and the success of the otherwise ornery and not always appreciated house-cat can probably be better attributed to our distaste for rodents than for our love of felines.

The immediate effects of rodents are pretty obvious: spilled rice grains on the shelf of the pantry reveal, on closer inspection, a hole chewed through the bag; small brown droppings foul the silverware drawer. Rats and mice are also notorious for their role in spreading diseases such as hantavirus,rabies, and bubonic plague. But because of their sheer number, rodents affect the world in ways that can be difficult to fully appreciate. In the 1980s James Brown (the ecologist, not the soul singer) of the University of Arizona conducted a series of experiments which showed that certain seed-eating rodents could transform a grassland into to a shrubland. Once Polynesian rats established themselves in Hawaii, they took to eating the seeds of native fan palms eventually wiping out forests in places like Ewa Beach on Oahu.

The uninhabited Northwestern Hawaiian Islands remained rodent-free until fairly recently.  Lisianski Island may have been the first to have been colonized when in 1844 a ship visiting from Honolulu deposited some mice onshore. Lacking natural predators, introduced rodents can increase in number at astonishing rates. Five decades after their first introduction, John Cameron, captain of the ship Ebon, made shore at Lisianski and found the island overrun.

We settled ourselves in for an appetizing supper of fresh food when myriads of mice attacked our meal ravenously and utterly without fear. Drive them away we could not; we slaughtered them by the hundreds, yet they would not be denied (quote taken from M Rauzon, Isles of Refuge)

Rodents didn’t find their way to Midway until decades later. Despite the establishment of the Pacific Commercial Cable Company Station on Sand Island in 1903 and the frequent supply ships that brought provisions to its residents, it was not until after WWII that rats and mice arrived, more or less at the same time, presumably on a Navy ship. By 1943 the black rat and the house mouse were noted as present. While the black rat eventually become established on all three of the atoll’s islands, mice were limited to just one, Sand Island.

The effects of rats on Midway’s wildlife were noticed pretty quickly.  Harvey Fisher and Paul Baldwin of the University of Hawaii visited the atoll in 1945 to see how birds were faring after the war. They were especially interested in two species: the Laysan rail and Laysan finch. Not indigenous to Midway, these birds had been brought there decades earlier from their native Laysan Island, where an overpopulation of rabbits had transformed their lush habitat into a barren wasteland, in what must have been one of the earliest wildlife translocation efforts ever attempted. Fisher and Baldwin were extremely disappointed to find that neither the finch nor the rail, which had thrived after their introductions, had made it through the war. The cause of the bird’s demise, they concluded, wasn’t bombs or bullets, nor could it be attributed to the habitat that had been lost to buildings, roads, and runways but rather it was due to a major infestation of rats. Sadly for the Laysan rail – and for us – its disappearance from Midway was tantamount to extinction; if you want to see one you’ll have to settle for an old grainy photo or a stuffed specimen in a museum. Laysan finches were able to pull through the tough times; rabbits were exterminated on Laysan Island in 1923 after which populations rebounded.

The Laysan Rail (Porzana palmeri) was introduced to Midway in the late 1800s and thrived until rats were introduced in 1943 after which it quickly went extinct. Top:  Illustration by John Gerrard Keulemans. Bottom: 1913 photograph by Alfred M. Bailey.

Rats had taken their toll on other bird species as well especially burrow-nesting species such as Bonin petrel, Bulwer’s petrel, wedge-tailed shearwater which are especially vulnerable. Numbers of Bonin petrels had been estimated at about half a million prior to the arrival of rats on Midway; Fisher and Baldwin counted only about 25,000, a 95% reduction. Rats not only preyed on the birds’ eggs but also attacked chicks and adults. Capable climbers, rats also impacted tree-nesting like the brown noddy.

As rats were a nuisance to people as well as wildlife, the Navy made some efforts to control their populations, but impacts to many seabird species persisted. Albatrosses, may have been less affected by rats than were other species, perhaps because their chicks are large and are guarded by a parent for some time after hatching. Which is not to say that it was smooth sailing for them during those times. Roadbuilding, mowing of lawns, overhead power lines, collision with aircraft, harassment by people and dogs, and the deliberate destruction of nests in areas where they were not convenient resulted in steep declines in their numbers, especially for Laysan albatross which inhabits the central portions of the islands. Although no reports of rats killing albatrosses had been made on Midway, there were highly credible accounts of rat attacks on Laysan albatross similar islands.  While visiting Kure Atoll in the late 1960s, Cameron Kepler of Cornell University observed Polynesian rats attacking and sometimes killing adult Laysan albatross.

[We] frequently  encountered  injured  Laysan  Albatrosses  and  noticed  dying  and  dead  adults  with  large  gaping  wounds  in  their  backs;  12  such  birds  were  found  in  the  1963-64  breeding  season… The  open  wounds  were  always  found  on  the  birds'  backs,  either  slightly  anterior  to  the  uropygial  gland,  or  forward  between  the  scapulae.  Small  holes,  one  to  two  inches  in  diameter,  were  occasionally  found.  At  this  stage,  wounds  were  sometimes  obscured  by  feathers,  and  the  injury  did  not  cripple  the  bird …  More  often,  sores  five  to  seven  inches  in  diameter  were  seen.  The  thoracic  cavity  was  often  exposed,  and  ribs  and  scapulae,  or  even  lungs,  were  visible  through  the  gaping  hole.  The  wings  drooped  when  the  bird  stood  or  walked,  as  a  result  of  severed  muscles.  Birds  that  had  wounds  on  their  rumps  often  limped  or  were  unable  to  walk.  The  injuries  were  often  infested  with  the  eggs  of  flies,  and  occasionally  harbored  maggots.  The  feathers  surrounding  the  hole  were  caked  with  blood,  and  the  birds'  bills  were  stained  from  probing  into  the  wound.  Birds  in  these  advanced  stages  rarely  survived  the  night  following  their  discovery (Kempler, 1967)

Could it be that the Polynesian rat is more likely to attack albatrosses than the black rat?  Or, perhaps, control of rats on Midway kept them numbers low enough such that they could subsist on other prey? I’m not sure we’ll ever know, but the rodent situation at Midway continued to harm seabird populations until the mid-1990s when, in anticipation of transfer of the atoll to the US Fish and Wildlife Service, rats were eradicated through a massive trapping and poisoning campaign. The last rat to be seen at Midway was in October of 1997.

The birds approved.  Bonin petrels, which had nearly been wiped out during the age of rats, returned to the atoll to nest once again.  While numbers are hard to come by, it is thought that over one million of these birds now breed on Sand Island alone. Rats on Midway may have been exerting strong effects on vegetation as well. After their eradication, the native shrub naupaka appeared to greatly increase in abundance. Rats, apparently, were eating more than just seabirds and as naupaka plays an important role in stabilizing dunes, rats may even, indirectly, have been accelerating coastal erosion on the atoll.

With rats out of the picture one might think Midway’s rodent problems had finally been solved. And so it seemed for a couple of decades. Though Sand Island still hosted a significant mouse population, they were thought to be primarily a nuisance to the human residents. During my first few days at Midway last spring, I was a bit surprised at how many mice scurried across my headlight beam as I rode my bike at night. It was also hard not notice the dozens of little black bait boxes placed outside of buildings. While there were certainly lots of conservation challenges to be addressed regarding the birds of Midway – ingestion of plastic, lead poisoning, invasive species, and entrapment hazards just to name a few – rodents weren’t high on the list.

During the fall and winter of 2015-6, exceptionally warm water in the Pacific created a particularly strong El Niño which, for this part of the world, means below average rainfall.  Black-footed and Laysan albatrosses arrived from the northern Pacific Ocean around Halloween to begin their long breeding cycle.  Things were going along pretty much as usual when the team of bird counters arrived in mid-December to do their annual albatross census. Then, two days before Christmas, several of the bird counters reported finding five adult Laysan albatrosses with bloody wounds on their necks and heads.  The cause of the wounds was a mystery. Albatross sometimes engage in fights that leave them bloodied and occasionally a peregrine falcon or other avian predator finds its way to the atoll and these possibilities were considered. Fish and Wildlife Service staff investigated the situation through a variety of avenues. On January 5^th, motion-sensing cameras set up alongside nests captured images that many had a hard time believing. Photos clearly showed mice crawling onto the heads and backs of albatrosses while they were sitting on their nests disappearing under their feathers. When those birds were examined, it was revealed that the mice were chewing through the feathers and skin and then eating their victim’s flesh. Both Laysan and black-footed albatross were affected though the former made up the vast majority of the casualties.

Some of the birds succumbed to infection while others abandoned their nests. Video footage revealed that albatross appeared to be annoyed by the presence of the mice but did not seem to know how to respond to the attacks. Perhaps this is not that surprising as animals such as albatrosses that have evolved for millions of years in the absence of mammalian predators may behavioral adaptations that would allow them to defend themselves. It is for this same reason that the island avifauna here and in places like the Galapagos Islands show little or no fear of humans. Watching the videos is painful. You want the albatross to do something, turn its head around and snap the mouse in half with its powerful, sharp bill. Instead the bird is agitated and confused and the mouse returns again and again, seemingly bolder each time, slowing chipping away at the life of a magnificent bird that might otherwise have thrived for decades in some of the harshest environments that our planet offers. It just seems so wrong.

Mouse attacks on albatrosses were first observed on Midway Atoll in 2015 during the early part of the nesting season.  Upper left: A motion-sensing camera captures a mouse crawling on the head of an albtross  sitting on its nest;. Lower left: An obvious and severe mouse-inflicted wound. Right: Sometimes wounds can be hard to detect; in this case wet, ruffled feathers hide a bite on the bird’s right shoulder. Photos  on left courtesy of US Fish and Wildlife Service. Right photo by RV Taylor.

The world’s largest colony of Laysan Albatross was under attack the Fish and Wildlife Service worked diligently with wildlife experts from other agencies and organizations to formulate an appropriate response.  The plan eventually hatched (OK, I know that’s a blatant and not very clever pun but I thought you might need a little levity at this point) was to try to reduce the mouse population in the areas where the attacks were occurring using a combination of traps and rodenticide. The strategy appeared to work and mouse numbers declined in the treated areas and fewer new albatross casualties (birds bitten/injured, dead birds, and abandoned nests) were reported by the field surveyors. Would the mice attack the chicks once they hatched?  No one knew, so it was with great relief in late January when after checking hundreds of nests with newborn chicks that no evidence of mice predation was revealed. When all was said and done though, the impacts were significant: 480 albatross were injured, 57 abandoned their nests, and 52 died. In areas where albatross were seen attacking mice, nests failed at twice the normal rate.

Scientists were dumbfounded as nothing like this had ever been observed in any of the Pacific Islands. What had precipitated the sudden change in the relations between albatrosses and mice on Midway? Maybe the El Niño-caused drought had caused severe food limitations? Or maybe it was a consequence of successful efforts in reducing the invasive weed golden crownbeard, which produces abundant seeds? And most importantly of all, would the situation repeat itself the following winter? There were many questions but few answers.

It turns out, though, that there was some precedent for mice attacking albatrosses, but you had to go to the other side of the world to find it. Gough Island, in the South Atlantic Ocean, is a globally significant seabird colony and hosts the largest colony of Tristan albatross in the world.  During the nesting season of 2000/2001 a substantial portion of albatross chicks died of unknown causes.  Ross Wanless and colleagues from the University of Capetown and the Royal Society for the Protection of Birds set out to find the source of the nest failures. As islands go, Gough Island is unusual in never having had populations of rats, cats, goats, or sheep.  The only introduced mammal there is the house mouse but few believed that they might be the cause. How could a tiny mouse take down an albatross chick that weighs over 8 kg (17 lb)? Using a combination of field surveys and motion-sensing cameras they discovered the cause. Mice!  In this case, however, the rodents were not attacking the adult albatrosses but the chicks. Mice attacked Predation on chicks by mice reduced halved the breeding success rate of these albatross during that season. Subsequent monitoring of the Tristan albatross population on Gough Island painted an even bleaker picture as attacks of mice on albatross seemed to be increasing over time. A team led by Delia Davies from the University of Capetown conducted a follow up study during the 2013/2014 breeding season and found that only 5 of 20 chicks they monitored fledged and of the 15 failures, 14 were due to mice.

"All attacks happened at night and the initial attack was always by a single mouse, which repeatedly targeted the same site, usually on the chick’s rump, clinging to its down and biting until the skin was penetrated. When displaced by the irritated chick nuzzling its wound, or vigorously shaking its body, the mouse would return to the wound within seconds. Chicks were seldom attacked by more than 1-2 mice at a time, although one chick had up to 9 mice attacking at once (Table 1). The behavior of the mice suggested that this was a well-practiced technique, as they generally climbed directly onto the nest mound and the chick without hesitation (Davies et al. 2015)"

The situation on Midway remains unique. On Gough Island, mice never were observed attacking adult albatrosses, only chicks. And at Midway the reverse is true. Putting these two facts together probably makes everyone a little nervous as it suggests that the situation at both of these seabird colonies could probably substantially worse should mice at either location discover there might be yet another source of food.

Given that mice and albatross had seemingly coexisted for over half a century on Midway, no one was sure what the 2016/2017 nesting season would bring. After the albatrosses arrived in the fall and began nesting, we waited and watched anxiously, hoping that the events of the previous year would prove to be an anomaly. On December 4, while out checking birds in areas where mice had attacked the previous year, Fish and Wildlife Service biologist Meg Duhr-Schultz found several bitten birds, removing any possibility that the events of 2015/2016 were some kind of El Niño-driven anomaly. Staff and volunteers were deployed over the next few days to survey other parts of the island and more attacks were discovered. In less than a week the area impacted by mouse attacks had exceeded the total area affected during all of the previous year. And the fact that the mouse attacks were noticed several weeks earlier was of real concern.  Again, the Fish and Wildlife Service had to quickly figure out what to do and the decision was made to take steps to reduce the mice populations in the affected areas. The large area that had already been affected meant that staff and volunteers had to log a lot of extra hours and additional resources were sent from Honolulu. The bird counters pitched in too with some remaining an extra couple of weeks to assist with surveys and treatment. So far the actions that the Fish and Wildlife have taken seem to be having a positive effect. The abundance of mice in the impacted area dropped sharply in areas where rodenticide was applied.

My primary role has been information gathering and analysis, including mapping out the areas affected, and the design of treatments using geographic information systems. This latter part is critical as the rodenticide that is being needs to be broadcast by hand within a predefined grid and must be done in strict accordance with procedures.

Caption: A dead albatross found during the attacks of 2016/2017 next to an abandoned nest (Photo: RV Taylor).

As of today, 48 areas totaling 10.5 ha (26 acres) of mouse affected area has been mapped and over 1,200 bitten birds have been discovered, 211 of which have died. Nearly one thousand abandoned nests have been documented. Mice may also be having impacts on other seabirds here but it would be more difficult to detect, especially for the burrow-nesting species.  Sounds kind of depressing, huh?  It is, but at least we’re not just sitting back and letting it happen. And although the numbers may seem large they need to be put into context. This year, the bird counters tallied nearly a million breeding Laysan albatross across the atoll, so there are still hundreds of thousands of birds going about their business, more or less, as usual. Add to that all the non-breeders who liven up the atmosphere here nearly 24 hours a day with their crazy dancing and “singing” and a group of tight-knit community of good people who recognize how important it is to laugh in the face of adversity.

Map of areas where albatross have been attacked by mice on Sand Island, Midway Atoll National Wildlife Refuge as of 15 January 2017.Areas with green borders indicate areas where only abandoned nests found but no bitten or dead birds. Red triangles represent individual dead or bitten birds.

Another bright spot is that there are plans underway to address the underlying cause of the mouse problems here at Midway.  As the only atoll within the Papahānaumokuākea Marine National Monument that still harbors lingering populations of invasive rodents, there have been plans to eradicate mice from Midway Atoll for some time. The discovery that mice are harming the albatrosses should only strengthen the case for their removal and expedite the project’s implementation. First steps were, in fact, taken just last November when a team of biologists and invasive species experts from the Fish and Wildlife Service and the non-profit organization Island Conservation visited Midway begin a study of the project’s feasibility.  Although eradicating mice from a place as big and complicated as Midway is challenging, similar projects have been successfully completed on other, and in some cases even larger, islands. With the first-hand knowledge I’ve gained on how damaging mice can be to wildlife, I think it would be great to be able to play a role in the eradication effort here.  Who knows, maybe there will be an opportunity? What I am sure of is that after this experience I’ll never look at Mickey Mouse the same way again.

Further reading

Brown, JH, and EJ Heske. 1990. Control of a desert-grassland transition by a keystone rodent guild. Science 250:1705-1707.

Davies, D., B. J. Dilley, A. L. Bond, R. J. Cuthbert, and P. G. Ryan. 2015. Trends and tactics of mouse predation on Tristan Albatross Diomedea dabbenena chicks at Gough Island, South Atlantic Ocean. Avian Conservation and Ecology 10(1):5.

Fisher, HI and PH Baldwin. 1946. War and the birds of Midway Atoll. The Condor 48:3-15.

Hess, SC and J D Jacobi. 2011. The history of mammal eradications in Hawai ‘i and the United States associated islands of the central Pacific.  Pp. 67-73 in Veitch et al. Island invasives: eradication and management. IUCN, Gland, Switzerland.         

Kepler, CB. 1967. Polynesian rat predation on nesting Laysan Albatrosses and other Pacific seabirds. The Auk 84: 426-430.               

Rauzon, MJ. 2001. Isles of Refuge: Wildlife and history of the Northwestern Hawaiian Islands. University of Hawaii Press. Honolulu, HI.

Wanless, R. M., A. Angel, R. J. Cuthbert, G. M. Hilton, and P. G. Ryan. 2007. Can predation by invasive mice drive seabird extinctions? Biology Letters 3:241-244.