Starving rats and mice prey on birds and reptiles after mega-mast food runs out

Monday, 16 December 2019

Broadcaster Jesse Mulligan sets traps in his backyard to catch rats and help save native birdlife. This video first screened in 2019.

Plagues of rats and mice are preying on native birds and reptiles now that seeds from this year's mega-mast have been exhausted.

Masting is the term used to describe a season of particularly high seed production among beech and rimu trees.

Half of all New Zealand's beech forests released their seeds this year, providing a 'bounty of food' for native birds, but the phenomena also provided food for rodents – enabling then to survive the winter.

Mice were able to survive the winter months due to an abundance of seeds from the mega-mast.

James Russell, associate professor at University of Auckland and a scientific advisor to Predator Free New Zealand, said the pests are 'coming out in the day and turning their attention back to native birds and reptiles'.

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'We are in the summer following the mast seeding event, when the rodents have survived winter with abundant food and not died-off, continuing to increase in population size,' he said.

'When pest numbers are highest they cause the most damage and they are high everywhere following mast years.

'Predator control needs to target the pests before the masting fruit and seed runs out and they switch to native birds and reptiles.'

Cutting-edge research is being undertaken by groups such as Zero Invasive Predators to entirely eradicate rodent populations from areas of the mainland.

Mega-masts do not always occur and depend on the difference between average summer temperatures in successive years.

If the second summer is warmer than the last, there's a high likelihood of a mast in the coming year.

University of Canterbury biological sciences Professor Dave Kelly said masts were normally expected every four or five years, but they were recorded in 2014, 2016 and 2019.

'Firstly, it's odd to have had three big beech [and] snow tussock mast years in six years,' he said.

'Secondly, this year seems to be quite widespread, which is driven by how widespread the very warm 2017-18 summer was.'

In November, Stuff reported that an infestation of mice in the mountain village of Arthur's Pass was 'the worst rodent plague in at least five years'.

Kelly said the central South Island can experience high mouse populations.

'If you talk to anyone with a bach in Arthur's Pass there are years when they are overrun with mice,' he said.

'In non-treated areas, mice are now peaking and running out of food. Rats, where present, are now at high levels. Stoat numbers are up greatly. Impact on native species is starting to rise.

'In 1080 treated areas, rodent numbers and stoat numbers are low and will remain low [and] birds are having a good breeding season.'