This rookery, containing chicks, was found in Mangakino.
The regional council believes rook numbers have fallen from about 50 to 40 in the region over the past year.
Introduced from Britain to control insects more than 150 years ago, the bird soon wore out their welcome.
Today the are considered one of the most destructive farm production pest birds in the world.
The Waikato Regional Council makes an annual appeal for sightings.
Biosecurity officer Andrew McConnell says three sites were found last year in Paeroa, Mangakino and Tīrau 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.”
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.
“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,” Andrew McConnell said.
“They’re also very wary so it’s important that landowners don’t try get rid of them themselves. We don’t want to scare them; it becomes tricky locating rooks once they have been spooked.”
Rooks build nests in pine or eucalyptus trees.
