309 mature individuals
Many of us are familiar with New Zealand’s variable oystercatcher, the black and white (or sometimes entirely black) wader with bright orange legs and bill that we encounter on beaches around the country. What most don’t know is that the Chatham Islands have their own version, the Chatham Island oystercatcher and that it narrowly escaped extinction in recent times.
The first comprehensive survey of Chatham Island oystercatcher in 1987 revealed 112 birds. Despite the low number this was actually good news because the previous estimate, made from partial beach counts in 1970, suggested that the population may be as low as 50 birds.
The reasons behind the perilously low population were investigated and found to be numerous.
Cats and grass
Ground-nesting birds are particularly vulerable to introduced predators and oystercatchers no exception. Monitoring of nests by video camera showed that cats were effective predators of oystercatcher eggs, chicks and adults. Weka, introduced to the Chathams’ in 1905, also took eggs as did rats, possums and hedgehogs.
Disturbance by domestic stock was also a significant issue, with both sheep and cattle either curiously investigating the incubating birds or inadvertently trampling the contents.
A further factor proved to be the loss of nests to high tides or storm surges, the risk which was significantly increased by marram grass. Introduced in the 1920’s to stop sand dune movement and protect farmland, marram has edged out other native dune species and covered the open areas in which oystercatchers prefer to nest. This forces the oystercatchers to nest close to the high-tide line and thus risk their eggs being swept away in a storm.
Between 1998 and 2004 oystercatchers on northern beaches of Maunganui and Wharekauri were subject to extensive management. Predators were trapped, stock access to the beaches was reduced and nests too low down the beach were inched inland to increase their chances of success. An area on Maunganui beach also underwent a transformation that saw the marram grass sprayed and native trees, pingao, forget-me-nots and other endemic coastal plants returned. The results were clear; breeding success improved from 0.37 chicks per pair in unmanaged areas to 1.04 in the managed areas. This increased productivity along with improved adult survival saw the population climb to 316 birds.
Change of focus
While the northern beaches do support the majority of the oystercatcher population, there is an important southern population centered on Pitt Island. In order to boost the population there, effort was moved from the north to Pitt in 2005, with cat trapping and nest manipulations carried out by the local ranger in an effort to improve breeding success. Unfortunately, these actions were largely unsuccessful with chick production only signicantly better in one year out of five. Most nest losses were to storm surges.
Meanwhile on Chatham Island, the large decrease in nest success alarmed conservation managers and predator trapping resumed on Wharekauri beach. Unfortunately this too proved to be too little as the very steep dune profile on much of the beach made nesting birds very vulnerable to storm surges.
A census in 2010 showed mixed results. While the overall estimated population had decreased only slightly to 309 birds, the decline in the number of juvenile and non-breeding birds (floaters) was significant. This was alarming as these were the birds that filled the gaps in the breeding population. It was feared that if the trend continued then the number of breeding pairs would soon begin to drop as replacement birds became unavailable.
The control of feral cats is important to the Chatham Island oystercatcher across its range, and conservationists and communities are working together to achieve it. Weka are a more difficult problem, being a valued food source, and despite their considerable impacts there is little agreement within the community on the need to control them in oystercatcher areas.
The fencing of coastal areas has resulted in the dramatic decline in the presence of stock on beaches and has enabled the ongoing restoration of the Tioriori site, but stock is still an issue in some places.
The biggest remaining challenge is in tackling the marram grass and enabling the long-term reshaping of dune systems to a more natural less-steep profile and restoring the endemic plants that hold it together.