Managing populations of the Australasian harrier (Circus approximans) to reduce passerine bird damage in vineyards

Vineyards around the world sustain significant economic losses due to grape loss and damage caused by frugivorous passerine birds, and while bird control methods are in place, their efficacy is limited and/or short lived. With the call for more sustainable agricultural practices globally, it would be advantageous to offer an ecologically based solution to the bird problem in vineyards, while further research and development into cheaper, more effective methods of bird control that does not create noise or disturbance to communities surrounding vineyards is required. The Australasian harrier, a native, diurnal New Zealand raptor, is the focal species of this project. With considerable numbers of harriers sited around New Zealand viticultural land, the aim of this project was to attract populations of these harriers into vineyards by providing them with an important food source – animal carcasses. The presence of harriers was expected to exploit the innate fear that pest passerine birds have towards raptors and provide an effective biological control aid that would provide an economically and environmentally sound solution to passerine bird induced grape damage as the passerines responded to the harrier rather than foraging on grapes. The Australian harrier was attracted to raised feeding tables in Canterbury and Wairarapa vineyards with supplementary food. Results indicated it was difficult to attain regular feeding from all tables set up. Some feeding table sites saw harriers feeding off tables regularly and intermittently, while at other sites no harriers exploited the tables. When presented with a two choice food test on feeding tables, comprising one-day-old cock chicks and rabbit pieces in the springtime, chicks (86%) were the harriers’ clear choice over rabbit pieces (14 %). During the summer season, there was no preference, with equal amounts of both baits taken. Where feeding tables were present, pest bird abundance decreased by 56 %, and grape damage also decreased by 59 %; however, these results were not necessarily linked only to harrier presence. While harrier numbers increased due to feeding tables, so did the number of other predators. A further trial without feeding tables where supplementary food was placed on the ground to attract all predators, showed an increase of predators in the treatment sites compared to control sites, with harriers and cats the most frequently observed. Pest passerine bird densities in the control sites were higher than the treatment sites. Raised feeding tables baited with animal carcasses are not necessarily a reliable method to encourage harrier feeding in vineyards. Several reasons may explain why this method may be unreliable. The best reason may be the motivation to feed off a novel object, i.e. the raised table was not sufficient because of neophobic tendencies for some harriers, and these were difficult to overcome. Alternative, easily accessible food sources were readily available in some landscapes and agonistic relationships with other species, who were frequently seen harassing harriers in study sites, may well have confounded attempts to achieve feeding off tables at all sites. Findings perhaps negate the need for any feeding tables, and supplementary feeding alone may be the key to attracting harriers and other predators into vineyards to achieve the fundamental goal of decreasing pest passerine bird numbers and consequent grape damage