Conservation assessment of the Variable Oystercatcher Haematopus unicolor

The Variable Oystercatcher Haematopus unicolor is a polymorphic species endemic to New Zealand. There are no recognized subspecies. Plumage varies, apparently continuously, from a black-and-white pied morph, through a series of intermediate stages, to an all-black morph. Where they co-occur, the different phases interbreed freely and randomly. The breeding biology of the species is outlined, but there have been few detailed studies. The species is non-migratory. Some juveniles remain at their natal sites and some disperse. Once paired and established on a territory, adults are typically site- and mate-faithful. There have been no detailed demographic studies, but preliminary data from one region suggest that the population has the potential to grow at about 4% per annum.

Variable Oystercatchers are almost entirely coastal in distribution. They are found around much of the mainland of New Zealand and its offshore islands, but are sparsely distributed in some regions. They have not been recorded from outlying island groups. Over the past 40 years, the population has increased rapidly and is currently thought to number about 4,500–5,000 individuals. Counts from different regions suggest that the increase has occurred throughout the range of the species. Like other oystercatchers in New Zealand, H. unicolor was previously shot for food, and legal protection is thought to be the main reason for the increase.

The species is ranked ‘Least Concern’ by BirdLife International, and ‘At Risk (Recovering)’ under the New Zealand threat-ranking scheme. The main threat to the species is currently believed to be predation, particularly of eggs and chicks, resulting in low average productivity. Adults are generally long-lived. Human activities in the coastal zone result in loss or degradation of habitat, with recreational activities on beaches causing high levels of disturbance in some areas during the breeding season. In the longer-term, the potential impacts of climate change on this and other coastal-breeding shorebird species are substantial.

Conservation-relevant research which needs to be undertaken includes a definitive resolution of the relationships of all Australasian oystercatcher taxa, long-term monitoring of increasing human impacts in the coastal zone, and assessment of the potential consequences for this and other coastal species of climate change. The Variable Oystercatcher is increasing in numbers and has a relatively low threat ranking; dedicated management is therefore currently not undertaken by national or regional government agencies, although a small proportion of the population benefits from management of other shorebird species. H. unicolor can however be managed successfully by community groups, and this activity should be encouraged.