Compared to the Firth of Thames and the Kaipara and Manukau Harbours, Tauranga Harbour is not a major body of water. It is, however, the largest enclosed estuarine habitat in the Bay of Plenty, and home to many bird species, some of which breed in New Zealand and some of which travel half way around the world to nest in the wilds of the Arctic tundra.
The harbour is shallow and about 35km in length. There is a narrow entrance at each end, with 23kmlong Matakana Island lying like a giant sand bar in between. Extensive dredging and blasting of rock at the eastern entrance has deepened the approach to the wharves at Mount Maunganui, now one of the country’s busiest ports. Once the tide recedes, less than half the harbour is covered in water, the exposed mudflats providing ideal feeding grounds for wading birds.
A decade ago, when I first began taking an interest in the birds of Tauranga Harbour, there were many places to be found where birds congregated with little disturbance. Sulphur Point reclamation (destined to become a new port facility) was in its early stages of development with a large sand island where godwits, white-fronted terns, variable oystercatchers, New Zealand dotterels and banded dotterels could roost or nest undisturbed.
The sand island is long gone, and most of the reclaimed area is now planted in lucerne. What remains in open sand is often invaded by trailbike enthusiasts, who criss-cross the area at high speed and deter all but a few birds from attempting to nest.
Ten years ago the ocean beach on Matakana Island was an almost continuous strip of broad beach with birds nesting virtually all the way from the Tauranga entrance to the Bowentown Heads. Breeding colonies of dotterels, oystercatchers, black-backed gulls and Caspian terns could all be seen along this stretch of sand.
Since then the beach has been eroded at both Tauranga and Bowentown ends, with hundreds of pine trees heaped up on the shore and some swept away up the coast as far as Waihi Beach. Nesting birds have been affected dramatically: banded dotterels have abandoned the island completely and New Zealand dotterels are having little success; most black-backed gulls now concentrate near the entrances and are highly predatory, taking most young of other birds.
Caspian terns still thrive—they are large, aggressive birds, and though nesting right beside their adversaries, the black-backed gulls, they have been able to slightly increase their numbers to nearly a hundred pairs. Variable oystercatchers are also strong enough to resist the unwanted attentions of hungry gulls, and are still able to raise young on more isolated parts of the island.
New Zealand dotterels are the most vulnerable of the coastal birds because of their restricted range and habitat. A few live on Stewart Island and the lower South Island coast, but the bulk of the population inhabits the sandy beaches and estuaries of the North Island north of East Cape in the east and Kawhia in the west.
They and the variable oystercatchers tend to nest late, especially in areas with much human activity. Often it is November before eggs are laid, and with an incubation period of 28-30 days young are hatching during the holiday period, with its associated threat of disturbance. Banded dotterels, which are smaller than the endemic species, are more birds of the stony river beds of the lower North and South Islands, but they will also frequent ploughed land and open country not necessarily close to water. In spite of introduced predators, banded dotterels are thriving in many places.
White-fronted terns and red-billed gulls still nest on islands off Mount Maunganui with some success, but their colonies on shellbanks near Bowentown have suffered heavily from gull predation in the last three years. Two hundred pairs of white-fronted terns fledged only 30 young in the 1987-88 season, while the red-billed gull colony of 60-odd pairs raised no chicks at all. Storms also take their toll of chick numbers.
Migratory waders do not have these problems, though they do have to run the gauntlet of hunters armed with mist nets and firearms as they pass through parts of Asia, and reclamation of tidal mudflats at stop-over points on their travels (and here in New Zealand) is reducing the size of their feeding grounds, and could affect their numbers in the future.
The three common migrant wading birds found on Tauranga Harbour all nest in Siberia and Alaska. Bar-tailed godwits and ruddy turnstones are believed to travel directly across the Pacific Ocean to Eastern Asia, while banding of red knots has proved that many of them travel via Australia, and probably on through South-East Asia. The distance these birds travel is phenomenal: to Eastern Asia across the Pacific is over 6000km.
Not all migrant waders fly north each autumn; the first year birds remain in New Zealand during our winter months, their numbers varying according to the breeding success of the previous year. On average, about 10 per cent of the birds overwinter here.
Tauranga Harbour has a summer resident population of about 6000 bar-tailed godwits, making them the most abundant bird in the area. Their greatest problem is finding somewhere peaceful to rest when the mudflats are covered at high tide. When tides are at their lowest there are numerous mudbanks and sandbars where they can shelter undisturbed by people, but spring high tides force them to higher ground, usually on shellbanks near Bowen-town or, on quiet days, the Sulphur Point reclamation.
Stormy weather will force them on to grazing land away from the harbour edge, but they prefer a clear field of view to guard against perceived predators like dogs and humans.
Second in numbers to godwits among the migratory waders are the ruddy turnstones. From October to April as many as 350 are to be seen, mostly at the Bowentown end. Dumpy black, white and grey birds with short orange legs, they are the most easily identified of the international migrants. Even their tinkling call notes are distinctive, usually uttered as they fly in perfect formation from feeding grounds to roosting site. The flock twists and turns in perfect synchronisation, appearing to change from white to black to white again depending on whether you see their undersides or their backs.
Red knots are also regular visitors to the area in the warmer months. Dull grey and white like the godwits when they first arrive, the 60 or so knots are frequently hard to find among the vast numbers of much larger godwits. Sometimes in late summer they will stop over in the area in large numbers, and I have seen a flock of as many as 600 during February.
Like the godwits, knots colour brightly during February, attaining almost complete breeding plumage by the time they leave in early or mid-March. Male birds are especially colourful with rich chestnut feathers to chest and underparts, and backs deep brown spangled with gold.
Turnstones also colour up before heading north. They leave some weeks later after becoming rich copper-orange on the wings with white patches forming on the crown.
Continuing drainage of small swamps around the harbour is affecting the numbers of many other wetland birds. Ten years ago brown bittern could be regularly seen in many places around the harbour. Now they appear to be restricted to areas close to Katikati, where their booming call at dawn is the main evidence of their presence.
The loss of small wet areas on farmlands has also affected the stilt population. Well over a thousand feed around the harbour during autumn and winter, but few remain in the district to breed in spring.
Autumn is the time of maximum numbers of other New Zealand breeding shorebirds. Hundreds of banded dotterels and South Island pied oystercatchers and up to a hundred wrybills converge on Tauranga Harbour at this time.
Wrybills and pied oystercatchers come from South Island breeding grounds, but the banded dotterels come from many different areas, including the Ashley River near Christchurch, Lake Rerewhakaaitu near Rotorua, the Rangipo desert and the Tukituki River near Napier.
Kingfishers are still surviving in some numbers in the district, despite predation by cats and dogs around settled areas. Most birds nest in clay banks, often close to the ground, where dogs dig them out and cats catch them as they come and go from the nest. Humans occasionally dig them out, usually by accident while searching for the source of the strange grating calls of the nestlings.
As insects become scarce inland during the cooler months, large numbers of kingfishers converge on the harbour during autumn to feed on mud crabs. It is not unusual to see dozens of them perched on the power lines crossing the harbour to Matakana Island as they wait for the tide to recede.
Increasing disturbance on even the most isolated beaches is making it difficult for shorebirds to nest unmolested. Nevertheless, Tauranga Harbour remains a great place for birdwatchers, especially those with boats. As you drift slowly down some out-of-the-way channel at low tide the birds can be seen at their most active: wading birds industriously probing the mud, terns diving for small fish in the confined waters, and shags swimming and swooping under the water for a catch.