PredationPredation

Predation

Predation by the Ships Rat, Rattus rattus, and cats continues to be the greatest threat to the parrot and the greatest threat to the island’s native vertebrate fauna.

The Black Rat colonised the island in the 1940s (Hicks and Preece 1991). Domestic cats were introduced to the island in the late 1700s and were established as feral by the 1830s. Rats eat eggs and nestlings, and predate on brooding females. Cats predate on fledglings and breeding adults. Nest sites are within a few metres of the ground and accessible to both predators. Predation by Black Rats was identified as the cause behind six of fifteen failed wild Green Parrot nests from 1983-1991 (Hicks and Preece 1991) and the cause of at least two of eighteen failed clutches in 1996 (cause of failure for 15 clutches was unknown) (unpubl. data). Female Green Parrots appear to be particularly vulnerable to predation by cats and rats as only the females brood the eggs and females are very reluctant to leave their nests (P. Stevenson pers.comm.) Juveniles face the highest risk of predation immediately after fledging and 3-5 weeks later, when they begin to forage independently (Davidson 1997). Juveniles have poor foraging skills and spend a large proportion of their time on the ground, consuming fallen exotic fruits (abundant in most of NINP). In contrast, native Bloodwood seeds are collected and consumed perched in trees (Davidson 1997). An analysis of Green Parrot numbers and breeding output by Innes (1995), indicated that the parrot population and the number of breeding attempts had increased since rat baiting commenced. Rats and cats have been a common factor implicated in the extinction of three Cyanoramphus subspecies on other islands.

Potential natural predators of the Green Parrot are the hybrid Norfolk Island Boobook Owl (Ninox novaeseelandiae undulata), the Marsh Harrier (Circus approximans) and Kestrel (Falco cenchroides). Hybrid descendants of the owl are the subject of a concurrent management program and the diet of these owls has been monitored since 1986. No Green Parrots or Crimson Rosellas have ever been recorded (P. Olsen pers.comm.) and thus owls are not considered a significant predator of Green Parrots. The Polynesian Rat (Rattus exulans) may take parrots but as there has been a long period of co-existence, it is considered unlikely to be a significant threat (Anon 1984).

Since 1988, 88 natural hollows have been modified to prevent access by rats. Fifty-five of these hollows have been used by Green Parrots, many repeatedly. Rat proofing prevents rat access above or below the nest from elsewhere in the hollow tree. Galvanised iron sheeting (camouflaged) is placed around the limb or trunk outside the hollow for at least 30 cm in all directions from the hollow entrance. Nearly all nest sites have been modified, so there are no data to quantify the effectiveness of this method by comparing with breeding output from unmodified hollows. Given the ‘critically endangered’ status of the species, experimentation to resolve this issue is considered too high a risk. Only one natural site is considered sufficiently rat resistant to not require rat proofing. As well as predator proofing, nest hollows are regularly “serviced” through the addition of rotted Nestegis sp. (family Oleaceae) pulp after each nesting. This soft material forms a “bed” within the nest hollow.