Assessing What's at Risk - Rare species continue to decline on GBI by John Ogden
Environmental News Issue 31 Spring 2013
The Great Barrier Island Environmental Trust was involved in three bird-monitoring projects with results published in 2013. Members assisted Amelia Geary (DOC) in a bittern survey in spring 2012, Asher Cook presented a report on his work on tomtits and kakariki on Hirakima (with financial support from the Trust), and John Ogden published the results a decade of counting dotterels on GBI beaches. The references are in the box below. In addition trustee Kate Waterhouse has been much involved in lobbying for better protection of black petrels while they are nesting on Hirakimata or fishing in the Hauraki Gulf.
Black petrel, drawn by Joseph Smit in 1896
– from the British Museum Catalogue of Birds.
All these birds are at risk from predators and/or disturbance by people and dogs while nesting. Geary estimated that the breeding population of Bitterns on the Island is down to three pairs maximum. They appear to boom and breed in the Kaitoke swamp, but young birds may move to Okiwi, where they are usually seen later in the summer and autumn. Cook estimated between 37 and 82 pairs of tomtits on Hirakimata, but possibly as few as 6 pairs of kakariki in the same area. Numbers of the latter are very hard to estimate due to their mobility. Ogden’s estimates of New Zealand dotterel averaged 48 birds (12—19 breeding pairs) in the breeding season and 56 after breeding when the local population may be augmented by birds coming from Coromandel.
New Zealand dotterel numbers have remained fairly stable on GBI since 2000. However, their individual survival (c. 3 years on average) is less than dotterels on the mainland (c. 6 years), and they are vulnerable to disturbance by dogs on their east coast breeding beaches. They leave their nests as soon as dogs come within sight, and as a consequence eggs get fried in the hot sun, or eaten by other predators. Young birds are also vulnerable to dogs, using precious energy reserves running away from them. These effects are not readily observed by dog owners, but have been convincingly demonstrated (see: Environmental News # 29: p 22).
New Zealand dotterel on
Whangapoua estuary Photo: J. Ogden
Black petrels are continuing to decline rapidly, now numbering about 1000 pairs on Hirakimata; while Cook’s petrels are probably attempting, but failing, to re-establish their numbers on Great Barrier following rat eradication on Little Barrier. Predation by feral cats is definitely a factor. The black petrel is unquestionable one of the Island’s icons. Now there is a DOC hut on Mt. Heale it is much easier to spend a night up there and see these magnificent sea-birds.