Farmers in the Waikato are once again being urged to keep an eye out for rooks, which the Waikato Regional Council describes as "one of the most destructive farm production pest birds in the world".
The council issued a similar warning to farmers in September, but after a number of rookeries were recently found abandoned, there are now fears the "wary, destructive farm pest" may be breeding.
Biosecurity officer Andrew McConnell said seven rookeries were reported to the council in September, right around the birds' breeding season, and two of those received targeted pest control.
However, five of those rookeries have now been abandoned, and McConnell said he was interested to hear from any farmers who had seen rooks on their property lately.
"The season seems to be later this year so there is a chance they still may breed," McConnell said on Wednesday.
He said it was important to control the birds as they were bad news for farmers.
"They feed on newly sown crops and destroy paddocks by tearing them up in search for grubs," McConnell said.
"A large rook population can completely destroy entire paddocks. It's definitely not something we want to be seeing here in our region."
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.
The birds were introduced to New Zealand in the 1860s to control insect pests. And the largest colony ever found in the country was nearly 1000 nests, the council said.
The Waikato Regional Council has been managing rooks since 2002 and the population in the region is now estimated to be less than 50 birds.
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.
McConnell said finding the birds is "a bit like looking for a needle in a haystack", meaning the council desperately needed help from farmers who spotted any rooks on their property.
"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."