Comparative breeding of South Island pied oystercatchers (Haematopus ostralegus finschi) on river terraces and surrounding farmland in Mid-Canterbury, New Zealand

Abstract

Historically, the South Island pied oystercatcher (Haematopus ostralegus finschi, Martens 1897) (SIPO) has mainly bred on braided river terraces in the South Island. Since the 1950's, increasing numbers of SIPO have been observed foraging and nesting on surrounding farmland. The primary objective of this research was to determine which of these two habitats, farmland or river terrace, was the most productive area to nest in. The secondary objective was to establish reasons why SIPO breed on farmland. A number of breeding parameters compared between populations of SIPO nesting on a river terrace of the South Branch of the Ashburton River and SIPO nesting on surrounding farmland, mid-Canterbury during the 1999 and 2000 breeding seasons. SIPO breeding on farmland were significantly more productive than pairs breeding on river terraces. On the farmland habitat significantly more chicks were fledged per pair and per egg laid and chicks had a higher survival rate during the rearing period. Farmland SIPO also laid their first clutches significantly earlier than did river terrace SIPO, and these eggs were both larger in both size and volume. River terrace SIPO breeding density (pair.ha⁻¹), however, was significantly higher compared to farmland SIPO. Earlier clutch laying, larger clutches, larger egg volumes, high egg hatching rates and a high number of breeding pairs per hectare are all considered to be indicative of a high quality habitat. No difference was found between populations for egg survival rates or chicks fledged per hectare. Modelling predicted that sufficient chicks were being produced in each habitat for both populations to remain sustainable over the long term although the farmland population was increasing at a faster rate. The size of the river terrace breeding population fluctuated considerably over the study period. It appeared that high river flows at the start of the breeding season decreased the number of SIPO pairs that attempted to breed. By contrast, the farmland population remained relatively constant and did not seem to be influenced by any of the weather parameters that were analysed. The reason(s) why SIPO initially expanded their breeding range could not be determined from this study. Improvements in the quality of farmland habitats in the 1940's and 50's are likely to be the reason for subsequent expansions and also the dramatic increase in the population size of this species. In order to extend current understandings of SIPO breeding behaviour, further investigation is required.