Assessing What's at Risk
- Rare species continue to decline on
by John Ogden
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
– 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