Estimates of adult survival rate for three colonies of Sooty Shearwater (Puffinus griseus) in New Zealand

Abstract

The Sooty Shearwater (Puffinus griseus), also known in New Zealand as tītī or muttonbird, is an abundant, long-lived, trans-equatorial migrant that breeds in colonies in the southern hemisphere during the austral summer and migrates to and forages in the North Pacific Ocean during the boreal summer. That populations of Sooty Shearwaters have been declining has been indicated by falling oceanic counts, and reduced burrow density and declining rates of chick harvest of Rakiura Māori muttonbirders. Accurate estimation of adult survival rate of Sooty Shearwaters is vital if we wish to use a demographic model to help understand the decline and manage the population. Mark–recapture studies were carried out at three breeding colonies: Taiaroa Head, Whenua Hou and The Snares, all in southern New Zealand. Analysis of the data generally involved the use of models in which both transience and trap-dependence were allowed for. In addition, birds were split into two groups, according to whether they had been caught on the surface or under an inspection hatch that was placed over a breeding chamber. Our estimates of the level of transience suggest that a large portion of the population at each colony is composed of juvenile and pre-breeding birds. Model-averaged estimates of the survival rate for resident birds were 0.917 (95% C.I.: 0.860–0.973) at Taiaroa Head, 0.966 (0.933–0.997) at Whenua Hou, and 0.961 (0.706–0.996) at The Snares. A combined estimate for The Snares and Whenua Hou is 0.952 (0.896–0.979). Our estimates are higher than earlier estimates of survival for Sooty Shearwaters and congeners, presumably because transience has been more adequately accounted for than in earlier mark–recapture models.