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Brazen rats ruin riverside picnics 

Rats down by the Waikato River are being kept well fed by people enjoying a leisurely bite to eat and leaving their scraps behind.

A mischief of rats is going rogue in the city.

With the promise of an easy feed from an abundance of rubbish bin leftovers, the natural flight response of Hamilton's riverside rats is being dulled.

The brazen little beggars at Hamilton's Swarbrick's Landing are out in broad daylight and have even taken to beckoning for scraps, standing upright on their hind legs while park users dip into their hot chips.

It's odd behaviour. Rats are fearful creatures and largely nocturnal but Landcare Research wildlife ecologist John Innes said they are adapting.

"Swarbrick's Landing is used for a lot of picnicking and people are leaving a lot of food around and Norway rats, the species people are seeing in the daytime, they respond really readily to food supplies like that," Innes said.

There is also no poisoning done in the area. It's too dangerous for users of the neighbouring Day's Park dog exercise area.

There are plenty rats there.

In a small stand of harakeke and cabbage trees, 10 metres long and wide, about 20 rats come out to feed on a handful of fried potato chips left scattered on the ground.

When sparrows flock to the chips, the rats duck for cover. When the birds fly off, the rats emerge.

A short walk away, next to the council-owned barbecue, a dog leaps around a low growing bush overlooking the Waikato River. He's unsuccessful in his hunt and leaves.

The danger passed and a few minutes later, the rats return.

Pest Controller Jason is surprised by the behaviour.

"Rats are normally nocturnal and to see they out during the day is unusual," Jason said. "If you see them during the day, usually they are a problem and quite a big problem."

"What that means is that there is a large population of them and they are competing for food and they are having to get out for the day and scavenge for food rather than at night time when they prefer."

Rats are neophobic critters - they have a fear of anything new or unfamiliar.

They have poor eyesight and will avoid anything in their environment - even food - until they feel safe.

They are a "huge problem", Jason said, especially near the river which is a migration super highway.

Typically, in Hamilton, he'll install a two month pest control programme for Hamilton home owners.

If you live by the river, it's year-round monitoring.

"[Two months] is fine for other parts of town but by the river, they are rampant so you just can't get away with that.

"As soon as you get the ones there and take the bait stations away, they just move in."

The Swarbrick's rats have become comfortable said Hamilton City Council parks and open spaces manager Sally Sheedy and this spring has seen an upswing in residents contacting council about it.

In response, Sheedy is introducing a new weapon in the war of the rodents - chocolate.

"We'll be using, in assistance with [Waikato] Regional Council, some Goodnature traps in that location," Sheedy said.

"They have nice chocolate sauce applied and the rats go for that and they are, obviously, killed in that trap but it doesn't pose an issue in terms of an actual toxic bait."

So if dogs find a rat carcass and eats it, they won't ingest poison, she said.

Council have started a pest control operation in 11 city parks - something they do on a six-monthly and yearly rotation.

Onukutara Gully, which runs from Porritt Stadium at Chedworth to Wairere Drive near Hukanui Primary School, is one of the areas to be hit but is a special case.

Predator-Free Hamilton have been radio monitoring the movements of ship rats and Norway rats in the gully and a toxin free trapping programme begins there on Saturday.

Monitoring found most rats have a range of about 100m to 150m and while there is no data on how many rats there are in Hamilton, Sheedy said "they are not very abundant".

"What you are seeing isn't an explosion in rat population, it is just the population that we have."

It's a big population, though, said Waikato Regional Council biodiversity officer Dave Byers and conditions are ripe for breeding.

"There are quite a lot of rats around at the moment and that's come about, mainly from the mild winters, the last two winters that we've had," Byers said.

And, like the beech and rimu forests in the South Island where rats are converging in epic numbers, Waikato is experiencing a 'seed mast' where an abundance of seed drops from flowering trees.

"If there is lots of food, there will be lots of breeding going on as well."

Rat facts 

A group of rats is called a mischief.

Norway rats can have up to 22 babies in one litter but usually they have 8 or 9 babies.

Rats constantly gnaw to wear their teeth down as their incisors keep growing at a rate if about 11-14cm in one year.