The effects of sheep on the breeding success and habitat of the short-tailed shearwater, puffinus tenuirostris (Temminck)

Abstract

Stock, including sheep, are considered deleterious to ground-nesting birds and have been held responsible for declines in numbers of P. tenuirostris breeding in Bass Strait. Investigations during 1965-68 attempted to study possible interactions existing between sheep and shearwaters on Big Green I., Furneaux Group, Tas. The island has been stocked with sheep for more than 100 years and during this period changes in vegetation and decreases in colony size have been noted. Exclusion of sheep from a portion of a colony did not increase hatching or fledging success, nor was there a decline in either the number of burrows or in burrow utilization, in the sheep-trampled areas. Indeed, sheep activity over colonies appeared to decline during the birds' breeding season and it was concluded that burrow damage was due mainly to the birds themselves, with major damage occurring in the fenced area during the breeding season. Comparison of burrow stability on Green Island with that in another colony, again suggested that birds were responsible for nest destruction but that renovation by the birds tended to nullify the importance of the damage. Though shearwater colonies were once considered to be more widespread on Green Island, experiments have indicated that the vegetation, induced by man and perpetuated by sheep, was not preventing burrow formation and expansion of colony areas. Sheep have not, therefore, been shown to be responsible for past declines and other factors will be considered later.