We’re on the front lines of the Save Our Iconic Kiwi initiative. Our ranger Tim and his team have been studying the population of kiwi at Shy Lake to find out how to best protect them from predators like stoats. They have captured a number of kiwi and put transmitters on them, and are now monitoring them through the breeding season to find out how well the adults and chicks survive without pest control.
The first clutch of this season’s tokoeka nests has finished at Shy Lake and seven chicks hatched in our monitored nests. As usual, we were down after dark in many layers of clothing, flask of tea by our side and head-torch at the ready, waiting for the young chicks to come out. We managed to get a radio transmitter on all seven, and our next task is to check up on them every few weeks and ascertain their fate.
So far it’s been a mixed bag. On my first check up of the season, I followed the signal from Sinbad Colby’s nest, only to find it coming from a rata growing out of the side of a bluff. I couldn’t reach the transmitter itself, but stoat poo and bedding material in a cavity in the tree told the story clear enough. A grim start, and across the valley Karen had a similar story when she found that Long John Silver’s chick had also succumbed. Better news was to follow though: following up on Filibuster and Fortuna’s chick, she found that it had left the nest and was roosting on its own deep in a burrow, several hundred metres from the nest. With no reason to handle it and confident it was still alive, she let it be. This is the first time in this project we have observed a chick leaving both its nest and its parents naturally. There’s a long road ahead for this chick to reach the safe weight of about 1 kg, but it’s made a good start. We seem to be seeing fewer stoats on the nest cameras this year – they’re still around, but not as many. The average lifespan of the chicks has also been longer, and it’s giving me hope that one dodge the stoats until the cavalry arrives with our 1080 drop in autumn 2020.
With this in mind, I thought we should take a closer look at the perpetrator of this carnage: Mustela erminea, the stoat. Most Kiwis will know that stoats are public enemy number one for many of our native species. And they really are amazing animals. Lightning quick and great climbers, they’re not afraid to pick on someone several times their own size with a quick, targeted bite to the neck. They hunt by sight, sound and smell. So many of our species, kiwi being a good example, are adapted to hide from bird predators which hunt by sight and mostly in daylight. But being quiet, camouflaged and nocturnal is little protection from a stoat when you a leaving a strong kiwi scent trail everywhere you go.
They have a few other traits that make them such a devastating predator. Living life in the fast lane, they maintain a body temperature a couple of degrees hotter than us and their hearts rattle along at 4 or 500 beats per minute. It takes a lot of energy to run this motor and an adult male has to eat about 23 % of its bodyweight every day, or even more for a lactating female. That’s about 5 mice – and when there aren’t many mice on the menu, it means a lot of birds disappearing from our forests. If a stoat finds more food than it can eat or digest right away, it’ll kill as much as possible then cache the rest in a crevice for leaner times. This is what we typically see with kiwi chicks. The chicks we’ve lost at Shy lake are a similar weight to a stoat, and usually I find the legs (and transmitter) stashed in the base of a tree or log.
With such high energy requirements, stoats tend to follow a boom and bust population cycle. In lean times, they can have 80 % mortality over their first year in beech forest systems. But if there is plenty of food, they have some amazing adaptations to capitalise. Almost as soon as a female has given birth in spring, a male will visit and mate with her again. While he’s at it, he’ll mate with all the female young in the nest, whose eyes may not yet have opened and who may be his daughters. This means it’s hard to find a female stoat who isn’t pregnant, and she’s ready to start a family or seed a new population wherever she settles. The eggs she carries through the year are fertilised, but the female delays implantation – they don’t yet develop beyond a tiny ball of cells. When winter begins to release its grip, the embryos will start to develop again. If there isn’t too much food around, only a couple will develop to birth, or maybe even none. But if there is plenty of food around – as in a beech mast year when rodent populations explode – a female has the ability to implant more of those embryos and can bear a dozen kits. Five times as many stoats, just like that.
In summer the kits will leave the nest and disperse across the landscape – and again they’re bloody good at it. Juveniles have been known to disperse 70 km in a fortnight, and stoats have swum 1.5 km across the sea to invade islands. It’s no wonder that they invaded the western corners of Fiordland within 30 years of their introduction to New Zealand. This dispersal is one of the reasons our Shy Lake 1080 operation is being timed for autumn: the juveniles should have settled down into territories by then, and so by waiting a couple of months we are planning to minimise reinvasion of the controlled area. Remember that 80% mortality in a non-mast year? We’re hoping that with crashing rodent population and sop less food around, most of the immigrating stoats or survivors after the 1080 drop will get mopped up by the winter natural mortality. Prolific feeders, indefatigable dispersers, relentless hunters: it’s little wonder that these guys present such a challenge. But it’s one we have to meet, if a few decades down the track we want to hear the call of the kiwi in our wild places, and maybe even in our back yards.
This is the twenty-third in a series of posts about Tim’s work, follow the Conservation Blog to keep up to date on his progress.