Conservation assessment of the South Island Oystercatcher Haematopus finschi

The South Island Oystercatcher Haematopus finschi is found throughout coastal regions of New Zealand, but breeding is confined to the South Island, and Hawkes Bay and Wairarapa in the North Island. Breeding used to be confined to gravel riverbeds, but in the past 60–70 years has spread onto farmland, and most recently onto coastal beaches. Most breeding adults leave inland nesting territories from December and move to coastal estuaries, with most settling in the northern South Island and northern North Island. Adults return to breeding areas from early June, but 16–18% of the population, presumably young birds, remain at estuaries throughout the year. On the coasts they occur on sandy, muddy and gravel intertidal flats and beaches although they will also use grassy fields. The clutch size is 1–3 eggs, with up to two replacement clutches being laid if earlier ones are lost. On farmland, trampling by stock, farm activities (such as cultivation) and unknown causes are the main causes of egg loss. The age of 1st breeding is 4–6 years and annual adult survival was 89% in the period 1987–1997, and so the oldest birds would survive >25 years.

In 1940 there were probably <10,000 birds, but since being protected from hunting the population increased to an estimated 49,000 by 1970–71, and then 112,000 in 1983–1994. Subsequently, numbers probably increased for a few more years, but then likely have declined. Changing land use within their core range enabled breeding birds to establish territories on farmland and this resulted in an increased population, eventually leading to an expansion of their breeding range south and north.

Since the arrival of humans in New Zealand, the main threats to these birds have been hunting (until 1940), the introduction of mammalian predators and invasion of riverbed nesting habitat by tall vegetation. Although land use changes initially provided new breeding areas, ongoing changes in land use may reverse this situation, with extensive areas converting from sheep to dairy farming. In addition, in their coastal wintering areas population abundance may be affected by introduced invasive plants reducing food and roosting space and continued expansion of urban dwellings and consequent rise in disturbance to feeding and breeding birds.