In Otago, the rook, a pest bird which damages crops, is confined to South Otago, Strath Taieri and the Maniototo, the Otago Regional Council says.
Observations by the council's regional services staff were that about 66 birds remained in the Strath Taieri area, 24 each in South Otago and the Maniototo, and about 15 birds at Waipahi on the Environment Southland boundary.
Regional Services group manager Jeff Donaldson said staff had been successful poisoning nests but now many birds were making nests and moving on.
"We're not having birds stay in nests in the same way."
They had had some success with laying poison in new grass paddocks in pasta or walnuts, but the older birds were "very intelligent".
A report to the regulatory committee recently said 17 nests in the Clarks Junction area had poison applied last November. About 50 birds were in the area but the success of the work would not be known until next spring.
No formal work was carried out in the South Otago or Maniototo areas last year because of the apparent failure of the rooks in laying eggs and rearing chicks, probably because of the previous control work creating a sex-biased population.
Birds had been building nests and then after a few weeks vacating the sites and drifting aimlessly and, in some cases, building a second wave of nests with no eggs being laid.
It was a common pattern throughout New Zealand where control work had been done, the report said.
