New Zealand Journal of Ecology (2015) 39(2): 245- 253

Predator control improves nesting success in Waikato forest fragments

Research Article
John Innes 1,*
Carolyn King 2
Scott Bartlam 1
Guy Forrester 3
Robyn Howitt 4
  1. Landcare Research, Private Bag 3127, Hamilton 3240, New Zealand
  2. School of Biological Sciences, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand
  3. Landcare Research, PO Box 69040, Lincoln 7640, New Zealand
  4. Landcare Research, Private Bag 92170, Auckland Mail Centre, Auckland 1142, New Zealand
*  Corresponding author

Predation at nests contributes importantly to current declines of New Zealand forest birds. We monitored the survival of natural and artificial arboreal nests in small forest remnants south-west of Hamilton, where ship rat (Rattus rattus) and possum (Trichosurus vulpecula) abundances were also being measured in Summer 2008/09. Artificial cup nests (N = 77) were placed in replicated blocks with and without pest control, in both December and January. Natural nests (N = 11, five bird species) were observed from 13 October to 23 December 2008 in a forest with no pest control. Digital video cameras identified ship rats, brushtail possums and harrier hawks (Circus approximans) as predators of eggs and chicks. There was no difference between artificial and natural nests in daily survival rates monitored in December in a block with no pest control, suggesting that artificial nests are reasonable surrogates for natural nests. Bite marks on clay eggs, other diagnostic sign, and DNA swabbed from real and clay eggs confirmed ship rats and possums were the major introduced predators at artificial nests. Bite marks also confirmed that harriers contribute to nest failure. Removal of ship rats and possums in December improved the 14-day probability of survival of artificial nests, from P = 0.63 (95% CI 0.45–0.77) in the non-treatment block to P = 0.88 (0.74–0.95) in the treatment blocks. In January, the 14-day probability of survival in all three blocks was intermediate at 0.80 (0.69–0.87), and the variation between them could not be explained by including pest control in the model. The abundance of ship rats apparently declined even in the non-treatment block over this time, for unknown reasons. Our data from tawa (Beilschmiedia tawa) forest remnants confirm that control of ship rats and possums alone is sufficient to improve nesting success of small arboreal birds in North Island forests.