Interspecific interaction and habitat use by Australian magpies (Gymnorhina tibicen) on sheep and beef farms, South Island, New Zealand

Abstract

Australian magpies (Gymnorhina tibicen) were introduced to New Zealand in the 1860s from their native Australia. They are regarded as a threat to native biodiversity due to their conspicuous attacks on native birds and some sheep/beef farmers actively control them by trapping, poisoning or shooting. However there is little evidence that magpies are seriously affecting other birds on New Zealand sheep and beef farms. We conducted distance sampling surveys of bird abundance across 12 ‘clusters’ of sheep and beef farms between 17 November 2004 and 31 January 2005 to test whether magpies affect the abundance, conspicuousness and habitat use of other birds. Clusters were spread between Marlborough and Southland in eastern South Island, New Zealand. Each had a ‘conventional’ farm (no accreditation scheme), a certified organic farm, and an ‘Integrated Management’ (IM) farm. The farms within each cluster were within 25 km of each other and approximately matched for altitude, rainfall and soil type. The study aimed to: (1) determine whether the presence of many magpies interferes with the conspicuousness and therefore the abundance estimations of other species; (2) test whether magpies reduce the abundance or even cause the localized extinction of some other species on farms or alter their habitat use within the farms; (3) advise sheep/beef farmers whether control of magpies is a worthwhile investment for biodiversity conservation on farms; (4) determine whether magpie abundance varies between conventional, organic, or integrated management farms; (5) describe the variation in magpies numbers and their habitat use, and (6) consider whether magpies should be a priority focal species for ongoing ecological monitoring of ARGOS farms.