Population estimates of burrow-nesting petrels breeding at the Nightingale Island group, Tristan da Cunha Archipelago

Nightingale is a group of three small, uninhabited islands in the central South Atlantic Ocean. The islands are free of introduced mammals and are largely pristine, supporting two endemic land birds as well as globally important populations of several species of seabirds. Seven species of burrow-nesting petrels are known to breed on the islands, including roughly 40 % of the world's population of Great Shearwaters Ardenna gravis. We estimated burrow densities by systematically searching for their burrows in 5x5-m quadrats across the main island in the austral summer of 2015. A total of 1 789 petrel burrows fell within the 75 sample quadrats with an average density of 0.95 burrows.m(-2), suggesting that upwards of four million petrels breed on the main island. Burrow densities and occupancy rates were extrapolated by species for each habitat type to generate population estimates: Great Shearwaters 2.34 million burrows (1.82 million pairs, 95 % CI 1.67-1.97 million); Broad-billed Prions Pachyptila vittata a minimum of 83 000 burrows (with many more pairs breeding in rock crevices, total estimate 100000-500000 pairs), White-faced Storm Petrels Pelagodroma marina 17 800 burrows (11700 pairs. 95 % CI 4 700-16 600), Soft-plumaged Petrels Ptervdroma mollis 12 100 burrows (estimated 8000-10000 pairs), Fregetta Storm Petrels F grallaria/tropica 6 600 burrows (estimated 5 000 pairs). Common Diving Petrels Pelecanoides urinatrix 3 900 burrows (estimated 5 000 pairs), and Subantarctic Shearwaters Puffinus elegans an estimated 1000 pairs. Although Great Shearwater burrow densities and occupancies were lowest in the areas historically used for exploitation of chicks and eggs (ongoing, but now monitored), these results suggest the great shearwater population on Nightingale Island has remained relatively stable since the first estimates in the 1950s.