Distribution, abundance and control of the rook (Corvus frugilegus L.) in Hawke's Bay, New Zealand, 1969–2006

Abstract

The distribution and numbers of rook nests and rookeries in Hawke's Bay, northern Wairarapa and Manawatu were recorded in 11 surveys undertaken between 1969 and 1986. The area inhabited by rooks, as indexed from the number of 10 000 m grid squares occupied in each survey, increased from 61 in 1969 to 102 in 1984. The total number of nests reached a peak of 12 424 in 1978 and fell to 5635 in 1986, and the mean number of nests per rookery fell from 73 in 1969 to 22 in 1986. The rook population thus changed over the 1969–86 period from a dense population centred in large rookeries in a small area, to a less dense population in small rookeries scattered over a much larger area. The geographic spread was mainly to the south of the Heretaunga Plains to north of Masterton. While these changes in distribution occurred concurrently with initial rook control, there is no direct evidence of a causal relationship. Agricultural practices also changed over that time, and the rook population had been spreading through Hawke's Bay for many decades. Ground baiting with the avicide DRC‐1339 between 1971 and 1985 poisoned nearly 86 000 rooks across their range in the study area. This reduced the estimated damage to crops and pasture south of Hastings from $98,000 in 1969 and $154,000 in 1970 to almost nothing by the 1980s, as indicated by complaints received by DSIR and the Pest Destruction Boards and anecdotal evidence. DRC‐1339 was also applied directly to rook nests in an attempt to eradicate rooks from northern Hawke's Bay between 2002 and 2006. While more than 50% of rookeries were destroyed each year using this technique, this reduction was largely offset by the establishment of new rookeries and the re‐colonising of extinct ones. Non‐target deaths during rook control were largely confined to black‐backed gulls (Larus dominicanus) and Australian magpies (Gymnorhina sp.). These species made up only 1% of the total kill, and were only recorded during ground‐based operations. While the Hawke's Bay rook population is now widespread, the low concentration of rookeries and smaller rookery sizes mean that rooks do not currently pose the economic threat incurred by individual farmers in the 1960s and 1970s. The current rook population structure does not warrant large‐scale control efforts.