The effects of introduced mice on seabirds breeding at sub-Antarctic Islands Ben J. Dilley Thesis presented for the degree of Doctor of Philosophy Town FitzPatrick Institute of African Ornithology DST/NRF Centre of Excellence Department of Biological Sciences, Faculty of Science University of CapeCape Town of June 2018 University Supervised by Professor Peter G. Ryan The copyright of this thesis vests in the author. No quotation from it or information derivedTown from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes Capeonly. of Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University Declaration This thesis reports original research that I conducted under the auspices of the FitzPatrick Institute, University of Cape Town. All assistance received has been fully acknowledged. This work has not been submitted in any form for a degree at another university. ………………….................. Ben J. Dilley Cape Town, June 2018 i A 10 day-old great shearwater Ardenna gravis chick being attacked by an invasive House mouse Mus musculus in an underground burrow on Gough Island in 2014 (photo Ben Dilley). ii Table of Contents Page Abstract ....................................................................................................................................... iv Acknowledgements .......................................................................................................................... vi Chapter 1 General introduction: Islands, mice and seabirds ......................................................... 1 Chapter 2 Clustered or dispersed: testing the effect of sampling strategy to census burrow-nesting petrels with varied distributions at sub-Antarctic Marion Island ...... 13 Chapter 3 Modest increases in densities of burrow-nesting petrels following the removal of cats Felis catus from sub-Antarctic Marion Island .................................... 41 Chapter 4 The effects of mouse predation on burrow-nesting petrel chicks at Gough Island .... 63 Chapter 5 Mouse predation affects breeding success of burrow-nesting petrels at sub-Antarctic Marion Island ........................................................................................ 79 Chapter 6 ‘Scalping’ of albatross fledglings by introduced mice spreads rapidly at sub-Antarctic Marion Island ........................................................................................ 95 Chapter 7 Synthesis of key results and the way forward ........................................................... 117 References ................................................................................................................................... 127 Appendix 1 The distribution and abundance of blue petrels Halobaena caerulea breeding at sub-Antarctic Marion Island ................................................................... 149 Appendix 2 The distribution and abundance of white-chinned petrels Procellaria aequinoctialis breeding at the sub-Antarctic Prince Edward Island .......................... 169 Appendix 3 Trends and tactics of mouse predation on Tristan Albatross Diomedea dabbenena chicks at Gough Island, South Atlantic Ocean ........................................................... 181 _________________________________________________________________________________ iii Abstract Seabirds play keystone roles as apex predators in marine ecosystems and also influence the ecology of terrestrial ecosystems where they breed. Seabirds are among the most threatened group of birds - almost half of all seabird species are known or suspected to be experiencing population declines with 97 (28%) of the 346 species currently classed as globally threatened and at risk of extinction. Introduced predators at oceanic islands where many seabirds breed account for the largest proportion of population declines, more so than incidental fisheries bycatch or degradation of their breeding habitats. Since few oceanic islands have escaped invasion, the problem is widespread, with the prime culprits being introduced cats Felis catus, rats Rattus spp. and house mice Mus musculus which depredate adult birds, chicks and eggs. Rats were widely introduced to thousands of islands and their catastrophic effects on seabird populations have been well documented. Mice are estimated to have invaded more oceanic islands than any other alien predator, but until fairly recently they were considered to have little impact on seabird populations. This thesis focuses on seabirds breeding at two large oceanic islands - Marion Island (293 km2) in the south Indian Ocean and Gough Island (65 km2) in the south central Atlantic Ocean. Both islands have mice as the sole introduced mammal. Of relevance to this study, however, is that the density of burrow-nesting petrels is much higher on Gough Island because Marion Island’s petrel populations were greatly reduced by cats, which were introduced in 1948 and eradicated by 1991. In the early 2000s, researchers on Gough Island identified mouse predation as the most probable cause of the high chick mortality of at least three species of seabirds, including the endemic Tristan albatross Diomedea dabbenena. Further research concluded that mice can be devastating predators of seabirds on islands where they are the sole introduced mammal, because in the absence of competition and predation from larger introduced species, mice can attain very high population densities, and resort to attacking seabird chicks mainly in winter when there are few other food sources. In 2003, the first mouse-injured wandering albatross Diomedea exulans chicks were found on Marion Island and in 2009 the first attacks on summer-breeding albatross chicks were recorded, but incidents appeared to be infrequent. Although mouse predation had been identified as a potentially serious threat to seabirds at both islands, further evidence was required on how many seabird species were being affected and to quantify the impacts. Field observations suggested a noticeable increase in levels of mouse predation at both islands, yet there was still no direct evidence of mice depredating burrow-nesting petrels at Marion. In this thesis I assess the impacts of invasive mice at both islands and establish pre-eradication baseline estimates for the burrow-nesting petrel populations at Marion Island. Burrow-nesting petrels are the most abundant seabirds in the Southern Ocean, yet their populations are poorly known compared to surface-breeding albatrosses because they are difficult to survey accurately. Extrapolation from density estimates can lead to large error margins, but these can be reduced with the development of repeatable, island-specific survey methods for long-term monitoring. This forms the basis of Chapter 2, where I test the effect of sampling strategy (random transect or systematic survey) on population size estimates of three burrow-nesting petrel populations at Marion Island. Systematic, island-wide surveys were appropriate to estimate the population sizes of blue petrels Halobaena caerulea (strongly clustered distribution - Appendix 1) and white-chinned petrels Procellaria aequinoctialis (moderately clustered distribution - Appendix 2) and but for the very iv widely distributed great-winged petrels Pterodroma macroptera I counted burrows within random transects and extrapolated burrow densities by associated habitat attributes to generate island-wide estimates. The systematic surveys required more effort, but resulted in more accurate estimates for species with clustered distributions, whereas the random transects required less effort but resulted in broad estimates with wide error margins which limits the ability to detect changes over time. In Chapter 3, I investigate how burrow-nesting petrel populations on Marion Island have recovered since cats were eradicated in 1991. In theory, the removal of cats as the superpredator, combined with endogenous growth and the potential for immigration from nearby mouse-free Prince Edward Island, could have promoted a multi-fold increase in petrel numbers over the last two decades. To investigate this, I repeated a burrow-nesting petrel survey in the north-eastern sector of Marion Island originally conducted by Mike Schramm in 1979 and assessed how burrow densities have changed compared to densities at the peak of the cat-era. I found that burrow densities have increased by a modest 56% since 1979. The recovery of summer-breeding petrels decreased with decreasing body size, and winter- breeding species showed even smaller recoveries, which is similar to patterns of breeding success at Gough Island where mice are the major drivers of population declines among petrels. Mice are the likely cause of the limited recovery of burrowing petrels at Marion Island. To assess and document the impacts of invasive mice at both islands, I installed infra-red video cameras into burrows and assessed breeding success with regular burrow-scope nest inspections of study colony nests at both Gough and Marion Islands (Chapters 4 and 5). The results show that mice can be very effective predators of burrow-nesting petrel chicks and to a lesser extent, eggs. The breeding success for winter breeders were lower than for summer breeders at both islands, and among winter breeders most chick fatalities were of small chicks less than 14 days old. Fatal mouse attacks on small chicks were video recorded for six