It's not exactly the same size disparity as the elephant terrified of the mouse, but it's getting there – and unlike that cartoon trope, this one's legit.

On Midway Atoll in the Northwestern Hawaiian Islands, it's albatrosses – those huge, far-travelling seabirds – that are being persecuted by mice, and scientists aren't entirely sure why.

Midway hosts the largest albatross colony in the world, serving as the most important breeding ground for Laysan and black-footed albatrosses; a few short-tailed albatrosses reside here as well. (Among the feathered tenants is Wisdom, a female Laysan albatross that ranks as the oldest known wild bird anywhere; she was first banded on Midway in 1956.)

Midway Atoll - Bird Sightings - Mar/Apr 2015

House mice, meanwhile, have been a part of the picture only since World War II, when they were unintentionally released on Sand Island – the largest of the atoll's constituent isles – along with black rats. The rats have since been exterminated, but the mice persist: the only non-native mammals on the atoll. Yet they weren't known to seriously trouble seabirds – until 2015, that is.

That nesting season, biologists noticed ugly, bloody wounds showing up on full-grown birds in one corner of Midway. As Rob Taylor, restoration ecologist with the National Wildlife Refuge Association, noted on that non-profit's website earlier this year, refuge staff were initially a bit mystified, wondering whether the injuries stemmed from albatross-on-albatross conflict or perhaps a vagrant peregrine falcon. Trail cameras, however, revealed the true, whiskery culprits.

At first blush, a house mouse might not appear the most threatening creature to a mature albatross, but the sharp-toothed rodents were doing some nasty damage.

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Trail-camera footage of a house mouse attacking an albatross. Images: USFWS

"The mice were chewing through the skin, into the muscle and fat, and even into the birds' body cavities," Holly Richards, a United States Fish & Wildlife Service public affairs officer, wrote in a recent post. "Most of the wounds were located on the backs of the birds' heads, on their backs, and in some cases under the wings."

Despite their significant size advantage – and visible displeasure – the big seabirds behaved basically like sitting ducks, if you will, in the face of these pipsqueak attacks. Midway's albatrosses didn't evolve in the presence of terrestrial mammalian predators, and appeared incapable of or unwilling to defend themselves.

"To see these docile, easygoing birds being eaten alive was pretty jarring," said Aisha Rickli-Rahman, crew leader of the USFWS's Midway Biological Programme, at the time the mouse crisis first came on the radar.

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USFWS staff examine a Laysan albatross with visible signs of predation. Image: USFWS/Flickr

In the face of the onslaught, some albatrosses succumbed to their injuries or associated infection; others abandoned their nests. But biologists weren't sure whether the 2015-16 mouse attacks, which remained relatively geographically confined, were a bizarre aberration or a grim harbinger.

"In that first year we had over 40 fatalities, and a couple hundred birds were impacted," Matt Brown, superintendent of Papahānaumokuākea Marine National Monument (which encompasses Midway Atoll National Wildlife Refuge), said in Richards' USFWS writeup. "We hoped it was a one-time occurrence driven by climate conditions." (A very strong El Niño climate pattern was in force during the winter of 2015-16.)

As it happened, the following nesting season saw a significant expansion of the rodent rampage. "From 2015 to 2017 there has been an exponential increase in mice attacks – both in the area on the island that is being impacted and the number of birds that [are] being preyed upon and dying from their wounds," Richards wrote.

Mouse preying on a Laysan albatross sitting on its nest. Image: USFWS/Flickr

While it's not at all clear why, after all these decades, mice began preying on albatrosses, refuge officials recognise it as a serious threat. "If mice started regularly preying on adult albatross, we knew the potential was there for this to become a great big problem, not just for albatrosses but also for the other species of breeding seabirds at Midway," Marine National Monuments of the Pacific Wildlife Biologist Beth Flint told Richards.

Mouse onslaughts on nesting albatrosses may be a new thing on Midway Atoll, but there's some precedent on other remote isles around the world: for instance, house mice have attacked (and "scalped") the fledglings of wandering, sooty, grey-headed and light-mantled albatrosses on Marion Island in South Africa's sub-Antarctic Prince Edward archipelago, and an ambitious eradication effort will soon be underway on Gough Island in the South Atlantic, where hordes of mice have hammered rookeries of the endangered Tristan albatross as well as other seabirds.

As the 2017-18 nesting season on Midway kicks off, the USFWS will be closely monitoring the latest on the mouse front – and aiming for eradication of the exotic rodents. According to the Friends of Midway Atoll NWR, the agency will soon release an Environmental Impact Statement, as such an effort requires, with a public comment period to follow.



Top header image: Pixabay