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Rooks, originally liberated in Auckland, Nelson and Christchurch between 1862 and 1873, are now established in Hawke’s Bay (28 rookeries), southern Wairarapa (1), Christchurch (19), Banks Peninsula (3) and near Peel Forest (3 rookeries). The Hawke’s Bay and Peel Forest populations are probably derived from subsequent liberations.

The existing rook populations are located on the eastern side of the country and mostly in districts where grain is grown (yellow-grey earths). The Hawke’s Bay population (1242 nests counted over 300 square miles) is less dense than the Christchurch one (1371 to 1806 nests over 100 square miles). The Christchurch population has increased from 1000 birds (one rookery) in 1925 to 7000–10,000 birds (thirteen rookeries) in 1947, and then remained at about this level, but with nineteen rookeries.

Control operations, especially shooting and tree-felling, have been important in causing a reduction in some large rookeries, the establishment of several small new ones, and a slow increase in the breeding range of the species, to the south in Hawke’s Bay and to the west near Christchurch. The restricted distribution and slow rate of spread are attributed to the behaviour of the species (gregarious habits and use of traditional nesting places), the restricted distribution of liberations and the nature of the environment (climate and land-use).

The Christchurch rookeries were all in eucalypts until about 1926, but later ones are mostly in pines, the change over following an epidemic in eucalypts; eucalypts remain the favourite nesting trees in Hawke’s Bay. Limited data are presented on breeding season, clutch size and the location of winter roosts.