Download

The mainland New Zealand species of oystercatchers have overlapping winter ranges, but their distribution within these ranges is very different. South Island pied oystercatchers (Haematopus ostraIegus finschi) are concentrated in large flocks on major harbours and estuaries. Variable oystercatchers (H. unicolor) have a scattered distribution with flocks never exceeding 150 birds. Black phase variable oystercatchers are numerically dominant to pied or intermediate phases throughout New Zealand, although the frequency of the former increases southwards. Intermediate-plumaged birds occur where black and pied phases are sympatric. The rare Chatham Islands oystercatcher (H. chathamensis) is restricted to those islands. In the breeding season, H. ostralegus finschi and H. unicolor are reproductively isolated by their mutually exclusive breeding dispersions. They are also largely separated in their winter ranges. Variable and Chatham Islands oystercatchers have probably maintained their present numbers over the past 100 years. The decline that occurred in the numbers of South Island pied oystercatchers over the period 1870 to 1940 was arrested by the prohibition of shorebird shooting in 1940, from which time a spectacular irruption has occurred. At the current rate of increase, the numbers of this species will probably come under density-dependent control in the next few decades. This density effect may be alleviated for some time if the species expands its recent tendency to remain inland at breeding localities throughout the year, and adopt a terrestrial mode of feeding.