Monitoring of breeding success in 2006/07 and 2007/08, and visits in December 2007 to assess levels of stoat predation and burrow densities were undertaken in order to assess the status and conservation of Hutton’s shearwaters 10 years after an intensive study.
Long-term (20 year) estimates of burrow density within the Kowhai Valley show an increase in five out of seven monitored sub-colonies, and an increase in overall density within this colony. Along with the discovery of a new area of burrowed ground within the Kowhai these results suggest the population of Hutton’s shearwaters has increased in this colony over the last 20 years, at an estimated average rate of increase of 1.7% a year. Burrow density data for Shearwater Stream is less robust, but does not appear to show a decline. Measures of predation rates in the Kowhai colony show no major differences in the numbers of adult shearwaters found on transects in comparison with the late 1990s and the recovery of shearwater carcasses from down burrows in the two recent seasons also does not differ from the late 1990s. Burrow occupancy levels in both colonies in 2006/07 (53-57%, or 66-70% if a correction factor is applied) are similar to the earlier study.
In marked contrast, breeding success in both the Kowhai Valley and Shearwater Stream was unprecedentedly low in the 2006/07 breeding season and low (although within the previously recorded range for the Kowhai) in 2007/08. In both years Shearwater Stream suffered consistently lower (12-22%) breeding success than the Kowhai, a pattern also reported from 1987.
Continued monitoring of breeding parameters is recommended, along with predator control within Shearwater Stream. Modelling of at-sea factors, colony size and predation rates may help to understand differences between the two colonies and inter-annual variation in breeding parameters.