Rook hotspots in Paeroa, from Te Poi to Matamata, Mangakino to Taupō, and in Hamilton

Rooks, which generally build nests in pine or eucalyptus trees, are one of the most destructive farm production pest birds in the world. Photo / Supplied

Thanks to landowners reporting sightings, the number of rooks in the Waikato region is now believed to be below 40.

With spring in the air, Waikato Regional Council is again asking landowners to report rook sightings for targeted pest control to prevent the wary, invasive, big black bird from becoming a problem on farms.

Waikato Regional Council biosecurity officer Andrew McConnell says three sites were located and treated in Paeroa, Mangakino and Tirau last year, thanks to the help of landowners.

"The largest rookery, in Mangakino, was made up of six birds.

"The largest colony ever found in New Zealand was nearly 1000 nests but, thanks to control, such groupings are a thing of the past. We've been controlling rooks here in the Waikato since 2002 when their numbers were around 200. After last year's control work, we believe it to be below 40."

It's a good time of the year to start seeing rooks because it's breeding season, which is when they congregate. In the Waikato, rook hotspots are in Paeroa, from Te Poi to Matamata, Mangakino to Taupō and in Hamilton.

Once rookery sightings have been called in, a drone is used to survey for the presence of eggs and/or chicks in nests to ensure only active nests are treated.

"Males are known to build nests regardless," says Mr McConnell. "Using a drone means we don't waste resources on empty nests, and it improves efficacy by getting the timing of control right.

"We rely on landowners to tell us where they are because it's a bit like looking for a needle in a haystack for them, especially as their numbers get down.

"They're also very wary so it's important that landowners don't try to get rid of them themselves. We don't want to scare them; it becomes tricky locating rooks once they have been spooked."

"They feed on newly sown crops and destroy paddocks by tearing them up in search for grubs. A large rook population can completely destroy entire paddocks."

Rooks were introduced to New Zealand in the 1860s to control insect pests.