Central Bird Exchange - Whangapoua Estuary and Okiwi Spit in Focus
by John Ogden

In November this year I took a group of visitors attending the “Love Birds” weekend to Whangapoua Estuary and Okiwi Spit. The spit encloses the most significant wetland ecosystem for waders and ducks on Great Barrier Island. In a few hours there we saw a large flock of bar-tailed godwits (94!), a small group of Pacific golden plover, nesting pairs of New Zealand dotterel and banded dotterel, variable oystercatchers, terns and gulls.

The Whangapoua ecosystem comprises three parts: (1) the swamp system; (2) the tidal mud-flats and mangroves, and (3) the sand-dunes forming the spit. The swamp system has been formed by sedimentation and plant growth filling the inner part of the estuary, and is zoned with various species of sedges and rushes, and manuka further back. Most of this area was open water when the first waka arrived here about 750 years ago. The present mud-flats are being colonized by mangroves as this in-filling process continues. The spit mostly represents sand accumulation since sea-level reached it’s present level c. 6000 years ago.   

The swamp system is mainly fed by freshwater streams and occasional very high tides. The dense vegetation cover is the home of bitterns, spotless crake and fernbirds, and a nesting area for pateke/brown teal. Sadly the last certain recorded bittern nesting here was in 19811 , and spotless crake have not been heard here for many years either, but both species are probably still present occasionally or resident in small numbers.

In contrast at low tide the estuarine mud-flats are covered in birds. As the tide enters and leaves the estuary every day it brings with it millions of marine organisms, which are fed on by worms and mollusks (formerly mussels and pipi, now mainly cockles) in the mud. These in turn form a rich food source for wading birds, which come here to fatten up again after breeding or before long overseas flights. In the winter there will be New Zealand dotterels, banded dotterels, variable oystercatchers (plus the occasional South Island pied oystercatcher), brown teal, and maybe a few long-legged godwits and pied stilts. The estuary is the main stronghold for brown teal on Great Barrier, and is actively managed by the Department of Conservation (DOC) to protect this species. Rarities such as sand dotterels, whimbrels and sandpipers may also be present, perhaps with a tiny wrybill or two. All these have been seen at low-tide from the DOC campground, though binoculars are essential for certain identification.

We can recognize three groups of birds feeding on creatures in the mud; (1) locals; (2) visitors from elsewhere in New Zealand, and (3) overseas migrants.

Locals are present all year, but their abundance varies seasonally. For example nearly all Great Barrier’s New Zealand dotterels, and many of the oystercatchers too, come to Whangapoua after breeding, and disperse again to their breeding beaches in September though a few stay and breed on the spit. There are usually approximately 60 New Zealand dotterels crouched against the wind on Okiwi Spit at high tide in the winter months.

New Zealand visitors are mainly from the South Island – pied oystercatchers, banded dotterels, and wrybills. The one or two pied oystercatchers are hard to distinguish from some pied versions of the variable oystercatcher. Banded dotterels in winter plumage are like small versions of winter NZ dotterels; on average there are 40 – 50 present in March and April (fig 1) but only a few pairs stay on to breed, when their black and orange chest bands distinguish them from New Zealand dotterels. Wrybill numbers have declined drastically – to zero in 2015. This is almost certainly a consequence of breeding failures on their nesting grounds – the braided rivers of the eastern South Island. As dairying takes more water from the rivers, the gravel bank islands on which they nest become accessible to stoats, cats and rats, and also get invaded by weeds such as lupins in which the predators can hide. (see: “Wrybills disappearing from Great Barrier”. Environmental News 34, Summer 2015).

The overseas visitors are especially interesting. Bar-tailed godwits or kuaka arrive in New Zealand usually in September, having flown non-stop from Alaska. This is the longest flight of any bird – over 11,000 km – taking about 9 days, without a stop for resting or feeding! (See also www.teara.govt.nz/map/9184, or google bar-tailed godwit migration). The Whangapoua flock usually arrives in October or November, and up to sixty spend the summer feeding on the estuary (fig 2). By the time they depart again in March or April, a few of them are in their fine orange breeding plumage. Although only a dozen or less, the Pacific golden plover follows a similar pattern, most arriving in early summer and departing in early April – some in their magnificent gold and black spangled summer plumage. The migration route of these birds – which also nest in Siberia and Alaska – is less well known, but some certainly cross the pacific, probably stopping off in Honolulu on the way! Other migrants such as curlews and whimbrels turn up occasionally, probably blown off course from Australia.

The best time and place to see these birds is near the end of the Okiwi Spit at high tide. Because the mud-flats are flooded the waders come onto the spit for a rest, usually grouped together by species. There are often flocks of white-fronted terns and a few much larger Caspian terns. The latter are endangered in New Zealand but a few pairs nest around Great Barrier. There are usually a few red-billed gulls and large black-backed gulls with brown juveniles too.

In conclusion, the Whangapoua estuary is a very significant feeding place for many birds, at risk from predators in New Zealand, or from development in nesting grounds or along migration routes. The number of New Zealand dotterel over-wintering there alone make it a site of international importance under the 1971 Ramsar Wetlands Convention (www. ramsar.org). Clearly the estuary and spit have ‘international significance’ for bar-tailed godwits and Pacific golden plover, not to mention our unique endemic wrybill – which has no overseas ticket.

ADDENDUM:
If you spend much time on Okiwi Spit you’ll find dead birds too – wrecked on the beach after storms and often subsequently beheaded by feral cats. Penguins, fluttering shearwaters and tiny diving petrels are the most frequent. Just out to sea is Arid Island (Rakitu), where these birds may be nesting. If only the Department of Conservation would rid the island of rats we could expect these birds, and many other sea-birds to form nesting colonies on the Island.

 References:
1 Ogle, C. C. 1981 Great barrier Island Wildlife Survey. Tane 27: 177-200. See also: Geary, A., Corin, S. & Ogden, J. 2012. Monitoring Report. Australasian Bittern. GRBO 22380.
2 Ogden, J. & Dowding, J. E. 2013. Population estimates and conservation of the New Zealand dotterel (Charadrius obscurus) on Great Barrier Island, New Zealand. Notornis 60: 210-223.

Environmental News Issue 35 Summer 2016