An inexpensive method for identifying predators of passerine nests using tethered artificial eggs

We identified nest predators of two European thrush and three European finch species in the central North Island, New Zealand, using artificial clay eggs in active natural nests. The acceptance of the artificial egg by females was 75%, with low rates of female egg ejection (7%) or desertion (7%). Due to high predation rates we could not confirm the acceptance of six (11%) artificial eggs before predation occurred. Of the 57 nests that received an artificial egg 30 were preyed upon. We were able to successfully identify predators from 18 (60%) nests by imprints left in the artificial eggs, with rats (Rattus sp.), Australasian Harriers (Circus approximans) and Australian magpie (Gymnorhina tibicen) accounting for nine, eight and one of the predatory events respectively. In the remaining twelve predatory events imprints were too faint or no marks were left on the clay egg for identification. This study successfully demonstrates the use of an inexpensive, seldom-used method for quantifying and identifying nest predators, with low rates of nest abandonment and high rates of predator identification. We believe this method could add valuable information for future studies of nest predation in New Zealand.