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The white-chinned petrel Procellaria aequinoctialis is one of the most frequently observed seabird species captured in fisheries bycatch, yet some populations remain virtually unstudied. The size of the breeding population on the sub-Antarctic Auckland Islands, New Zealand, is unknown. Disappointment Island is thought to be the main white-chinned petrel breeding site in the Auckland Islands, and maybe also in the New Zealand region, and has never had introduced mammalian predators. We estimated the white-chinned petrel breeding population size taking into account the detection probability of burrows via distance sampling and the burrow occupancy rate. Eighty line transects were distributed over the island, with a total line length of 1600 m. Burrows were patchily distributed and most abundant in dense megaherb communities. White-chinned petrel burrow density $$ \hat{D } $$ was 654 burrows/ha (95 % CI 528–809 burrows/ha), with burrow detection probability $$ \hat{p} $$ varying among vegetation communities from 0.28 ± 0.02 to 0.43 ± 0.02 (±SE). Mean burrow occupancy was 0.73 ± 0.03. We document an estimated total of 155,500 (125,600–192,500) breeding pairs of white-chinned petrels on Disappointment Island during mid incubation in early January 2015. The relatively high occupancy and density of burrows suggest that Disappointment Island is a key breeding site for white-chinned petrels.

Major population changes of marine mega-fauna are ongoing as global warming, and other anthropogenic drivers affect prey availability. The historical stronghold of the Eastern Rockhopper Penguin (Eudyptes chrysocome filholi) was New Zealand’s sub-Antarctic Campbell Island, but the population declined by 94 % between 1942 and 1984. The apparent mechanism of collapse was warm ocean temperatures causing an inadequate food supply. Eudyptes penguin population declines are ongoing at some breeding sites, highlighting the need to investigate the population trend on Campbell Island since 1984. We estimated the Eastern Rockhopper Penguin breeding population size through physical and photo-counts of birds and nests in 2012, and changes in colony area relative to 1984 and 1996 photographs. We estimated the 2012 population size at 33,239 breeding pairs, a 21.8 % decrease from an (adjusted) estimate of 42,528 pairs in 1984. Although substantial, the recent 1984–2012 decline occurred at a much slower rate (λ = 0.991) than the 1942–1984 decline (λ = 0.940). Despite great variation in trends between colonies ostensibly linked to differences in predation rates, the recent decline occurred primarily between 1984 and 1996, and thereafter the overall population grew. A 100-year time series of extended reconstructed sea surface temperatures (ERSST) confirmed that the population declined during warm periods and increased during cool periods, but that the initial decline began before increases in regional ERSST. Population growth after 1996 appears related to the current global warming hiatus, lower ERSST, and increased abundance of a key prey species. We predict a continuation of the long-term population decline after warming resumes.