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Predation at North Island Robin Petroica australis longipes and North Island Tomtit Petroica macrocephala toitoi nests was studied in New Zealand over the 1993/94 breeding season to determine impacts of predators. Infra-red, time-lapse video photography and sign left after predation were used to identify predators at nests. Accurate estimates of predation rates depended on early detection of nests. Previous studies of predation may have greatly under-estimated predation rates and therefore predation impacts. Predation was patchy and intense, resulting in failure to produce young in some territories despite up to ten nesting attempts. A maximum of 82% of nests were preyed on (n = 65; 95% confidence interval 72.4%-90%) and Ship Rats Rattus rattus were probably responsible for at least 72% (95% confidence interval 57.4%-84.4%) of predations. Nine of 24 territories lost breeding females, mainly to Ship Rats, which significantly impacted on population productivity. Ship Rat predation was equally intense at exposed and concealed nests (at the site and patch levels). Predation attributed to avian predators was strongly correlated with exposed nests (at the patch level). Restoration of New Zealand's threatened forest bird communities is dependent on a commitment to further research into the significance of different predators and predation impacts on bird populations.
Key words: Nest Predation, Nest Concealment, Community Restoration, Rattus rattus, New Zealand.
INTRODUCTION
PREDATION by introduced mammals (Atkinson 1978, 1985; Bell 1978; King 1984; Holdaway 1989), including humans (Cassels 1984; McGlone 1989; Steadman 1989), is probably the main reason for the recent wave of avian extinctions throughout the Pacific. In New Zealand, the trend towards reduced diversity and densities of native avifauna continues with declines in at least 12 species or subspecies of rare forest birds (Inncs and Hay 1991).
Introduced mammals such as Ship Rats Rattus rattus, Brushtail Possums Trichosurus vulpécula, Stoats Mustela erminea and Cats Felis catus, and native avian predators, such as Ruru or Morepork Ninox novaeseelandiae and Kahu or Harrier Hawk Circus approximans, are known to prey on forest birds. Although Predation is considered to be the main cause of ongoing declines, the significance of different predators to forest bird populations has not been clearly identified or measured. In addition, past assumptions and methods of predator identification require reevaluation (Major 1991; Brown et al. 1993).
Accurate estimates...