New Zealand Journal of Ecology (1999) 23(2): 149- 159

Costs and benefits of aerial 1080 possum control operations using carrot baits to North Island robins (Petroica australis longipes), Pureora Forest Park

Research Article
R. G. Powlesland 1
J. W. Knegtmans 1
I. S. J. Marshall 2
  1. Science and Research Unit, Department of Conservation, P.O. Box 10420, Wellington, New Zealand
  2. Pureora Field Centre, Department of Conservation, R.D. 7, Te Kuiti, New Zealand

Large scale aerial poison operations with 1080-carrot baits are used extensively to control possum (Trichosurus vulpecula) populations in New Zealand forests for ecosystem conservation purposes and to stop the spread of bovine tuberculosis. Although various procedures have been implemented to reduce the incidence of bird kills, dead birds continue to be found after poison operations. Colour-banded North Island robins (Petroica australis longipes) were monitored in treatment and non- treatment areas to determine the costs and benefits of aerial 1080-carrot bait operations to robin populations. An August 1996 operation, that included much chaff or fine particles of bait, resulted in 43% mortality of territorial birds (banded and unbanded or 55% mortality of colour-banded birds. During the same period there was no robin mortality in the non- treatment area. By comparison, the September 1997 operation, that included minimal amounts of chaff, resulted in just 8.6% and 9.7% robin mortality as determined by territory mapping and of banded birds, respectively. These levels of mortality did not differ significantly from that evident in the non-treatment area. Monitoring of possums and rats indicated that both populations were at very low densities and remained so during the robin nesting seasons (September-February) following the operations. Robin breeding was particularly successful in the treatment areas following the two poison operations (mean of 3.7 and 3.8 fledglings per pair) compared with that in the non- treatment area in 1996-97 (0.4 fledglings per pair) and in the post-treatment area in 1997-98 (1.5 fledglings per pair). Through recruitment of the fledglings, both populations had more robins and a greater proportion of females one year after the possum control operations than immediately before. Thus the results suggest that as long as carrot bait protocols are strictly adhered to and baits are distributed over large blocks of forest so that mammalian predator populations remain low during the next robin nesting season, the robin populations will benefit from aerial 1080-carrot possum control operations.