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Rats on the Rampage
Judy Gilbert reports from the mountain top


North Island Robins through two breeding seasons I can see why they disappeared over a hundred years ago from Great Barrier. The rats certainly love (or hate!) them – both eggs and juvenile chicks. This season 13 eggs and 2 banded chicks were predated by rats despite intensive protection – nests surrounded by traps, poison baits, and often the tree banded to stop rats climbing up to the nest. Rats usually eat all the egg leaving shell remnants and sometimes only eat the brain of a juvenile bird. If this is the level of impact on birds that are being watched over and ‘protected’, then it can only be full scale slaughter that occurs every night over the breeding season on fantails, silvereyes, tuis, and greywarblers. In the recent GBIs Trust summer lecture on the impact of rats in the bush, Landcare research scientist John Innes’ analysis of bird predation laid 58% of the bird mortality on rats over all other predators. They are highly successful hunters and very thorough in covering all eating opportunities in their territories. There is not a single place on our Island that a rat cannot get to.

The photo the GBIs Trust has currently on display as a poster around the Island was taken by David Mudge and shows a ship rat eating juvenile fantails. David was undertaking a study on fantails and had set up a night vision camera to record nesting. The rat visited that particular nest some four or five times over the night before consuming both young. While fantails still appear to be fairly abundant they have to lay up to five eggs per clutch in 4-5 nests per season to keep their numbers up under the onslaught of rats. On occasion rats will even devour the nesting adult.

Over the last two years we have had mild winters and summers with enough rain to ensure high fruit and seed abundance in the bush. The seventy monitoring tunnels at Windy Hill showed an unusually high number of rats present last winter. I would say that this autumn we will all experience serious rat problems as their numbers have built up with the warmer winters and food abundance. While birds and insects also benefit from these conditions the rat numbers generally swell to such densities with all that food abundance that the small bird populations can be hugely impacted. Rats are highly adaptive and will breed more frequently and have larger litters as food availability increases. One of the main problems with rats is that not only do they predate eggs, birds, invertebrates, worms, and lizards but they also out-compete other species for the same food. I have heard several scientists say that in the presence of high rat numbers both kiwi and feral pig weights are lower – a result of reduced food avail-ability. Of the all the monitoring undertaken here on the pest projects at Windy Hill and Benthorn Farm the most significant difference shown between bush managed for rats and bush not managed for rats is in the seedlings. In rat reduced bush, seedlings are more plentiful and there is more species variety – in unmanaged bush there are less seedlings and some species are eaten out completely. While this is probably the least obvious effect of rampaging rats it has a profound effect on the future regeneration of the forest and the subsequent food and habitat availability for all bush dwellers.