Crossland A. C. (2001). Long term changes in numbers of variable oystercatcher (Haematopus unicolor) at two wintering sites in Canterbury, South Island, New Zealand. The Stilt, 40, 2-6.
When this paper was written in 2001, the population of variable oystercatchers in Aotearoa New Zealand was believed to have doubled since the 1970s. Wintering numbers of variable oystercatchers have been regularly monitored by researchers and members of the Ornithological Society of New Zealand at the Avon-Heathcote Estuary since 1947 and Lyttleton Harbour since 1985. This paper used data from such observations to demonstrate increases in variable oystercatcher numbers in Canterbury.
Overall, variable oystercatcher numbers on the Avon-Heathcote estuary were relatively stable at 1-4 birds over the first 40 years of monitoring (1947-1987). Numbers then increased steadily over the next 13 years to an average of 70+ birds wintering on the estuary. This increase was significant because in 2001 70+ birds represented almost half of the total Canterbury regional population and about 2% of the world variable oystercatcher population, technically qualifying the estuary as an internationally important site for the species under Ramsar Convention criterion. At Lyttleton Harbour, wintering numbers showed a similar increasing trend to the estuary. However, Lyttleton numbers were lower and started to increase later, indicating a ‘spill over’ effect from the estuary. Variable oystercatchers were absent or scarce on Lyttleton Harbour between 1985 and 1996, and peak numbers then increased to 16 birds in the year 2000.