Quick Dips

A Census for the Birds

Frances Schmechel

A census for Chatham Island oystercatchers (Haematopus chathamensis or torea) recently saw a group of 35 Department of Conservation staff, volunteers and local residents range along 310 kilometres of coastline on four of the islands in the Chathams.

By foot and by quad, they covered some 97% of the coastlines of the main Chatham, Pitt, Mangere and Southeast/Rangatira islands, as well as the shores of the Te Whanga lagoon, counting 141 adult oystercatchers, including 34 confirmed breeding pairs and seven possible breeding pairs. This is about 20-40 more oystercatchers than in any previous count. Seventy percent of the breeding pairs were on Chatham Island; 15% were on Pitt.

The majority of oystercatchers (79% of individuals and 74% of the breeding pairs) were located in areas broadly defined as associated with rocky, wave-cut platform or other rocky coastline or outcrops; though 30 individuals and nine breeding pairs were located along sandy beaches (or, in one case, the lagoon edge). Only one oystercatcher was seen along Te Whanga Lagoon; all others were seen along the coastline.

Chatham Island Oystercatchers are endemic to the Chathams archipelago, are non-migratory, and almost strictly coastal in their distribution. Breeding pairs are fairly sedentary and display strong territorial defence behaviours during the breeding season. As with most other oystercatcher species, they do not begin breeding until at least two years of age. The birds live up to 30 years.

The Chatham Island oystercatcher is listed internationally as endangered, and is one of DOC's "category A" species, or highest priority for conservation action. Past counts between 1986 and 1996 have estimated the population to be from 65 to 120 adults, including 30-44 pairs.

Because only one or two people conducted these counts, they occurred over limited areas or over relatively long periods (6-13 weeks), increasing the likelihood for under-counting or double counting birds, and missing areas where birds might be located. The December census was the first to be conducted over all four known breeding islands in the relatively short time-frame of one week.

DoC has been carrying out rescue measures to try to boost the population, fencing nesting areas off from livestock and removing predators such as cats, possums and weka. These are considered stop-gap measures until more accurate data about the birds' habitat use and breeding biology is gathered.

Little information exists regarding the historic abundance and distribution of the oystercatcher, with consequent important issues for conservation. It is possible that the species has always been rare and intervention may not be required, but it is equally possible that the population is a reduced relic from historically higher numbers. The latter is considered more likely given the environmental changes which have occurred on the Chathams since the arrival of humans.

Frances Schmechel is a student in the Entomology and Animal Ecology Department at Lincoln University