Waikato Regional Council is asking landowners to report rook sightings for targeted pest control.
It's a good time of the year to prevent the invasive big black bird from becoming a problem on farms, as it is breeding season, which is when they congregate together.
In the Waikato, rook populations are found in Te Poi/Matamata, Cambridge/Hamilton and Mangakino to Taupō, with rooks generally building nests in pine or eucalyptus trees.
Waikato Regional Council biosecurity officer Andrew McConnell said rooks were one of the most destructive farm production pest birds in the world.
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"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. It's definitely not something we want to be seeing here in our region."
Rooks were introduced to New Zealand in the 1860s to control insect pests. The largest colony found in New Zealand was nearly 1000 nests but, thanks to control, such groupings are a thing of the past.
The regional council has been managing rooks since 2002 and the Waikato population was now estimated at less than 50 birds.
"It's a bit like looking for a needle in a haystack for them, which is why we need landowners to give us a call when they see rooks" McConnell said.
"They're also very wary so it's important that landowners contact us if they see them rather than try get rid of them themselves. We don't want to scare them; it becomes tricky locating rooks once they have been spooked."
Last year, five rookeries were found and controlled: three in Mangakino and two in Matamata. A rookery in Hamilton was destroyed by a storm and abandoned.