The effects of introduced mice on seabirds breeding at sub-Antarctic islands, a PhD thesis by Ben Dilley

Ben Dilley off the leeward coast of Gough Island, Churck Rock on the horizon

ACAP Latest News has regularly reported on the attacks on seabird by introduced House Mice on both Gough and Marion Islands and the research conducted to understand the problem and help direct planned eradication efforts (click here).  During separate year-long expeditions to both islands much of this research has been conducted by Ben Dilley, then a PhD student (now a Post-doctoral Fellow) of the University of Cape Town’s FitzPatrick Institute of African Ornithology.

Ben completed his thesis in 2018; with his Doctorate awarded in 2019 it is  now available online.  Most chapters are based on his published papers, that also have been featured on the ACAP website.  The thesis abstract follows:

“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 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 super predator, 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 burrow-nesting petrel species and winter breeders had very high chick mortality rates (e.g. 82–100% on Gough Island). Since mouse depredation of seabird chicks was first identified as a problem in 2001, the frequency and severity of mouse predations appears to have escalated on Gough (Appendix 3), yet on Marion Island detected incidents remained infrequent until 2015, when mice attacked 4.0–4.6% of the large chicks of all three albatross species that fledge in autumn. Attacks started independently in small pockets all around the island’s 70 km coastline, separated by distances hundreds of times greater than mouse home ranges. Attacks have continued from 2016–2018 at varying rates on summer-breeding albatross fledglings, showing how mice alone may significantly affect threatened seabird species (Chapter 6). In summary, mice appear to be suppressing the productivity of burrow- and surface-nesting seabird populations at both islands and are very likely causing population declines, especially among winter breeding species. Fortunately, the removal of invasive mice from islands through aerial spreading of toxic bait is a viable option and the scientific and visual evidence collected during this thesis has contributed to the growing body of evidence needed to persuade funders and Governments to support eradication operations at both study islands.”

Following nocturnal attacks by mice on Gough Island, a Tristan Albatross chick has only hours to live, photograph by Karen Bourgeois and Silvain Dromzee

With thanks to Ben Dilley.

Reference:

Dilley, B.J. 2018.  The effects of introduced mice on seabirds breeding at sub-Antarctic Islands.  PhD Thesis, Department of Biological Sciences, University of Cape Town.  195 pp.

John Cooper, ACAP Information Officer, 25 March 2020