Above photo: Oceans and Coasts Ornithological Field Assistant Andile Mdluli deploys a trail camera at a Great-winged Petrel burrow, photograph by Samuel Peta
Mouse-induced mortalities of seabirds breeding at Marion Island continue to be a significant conservation concern. Since their accidental introduction in the early 1800s, the invasive House Mouse Mus musculus has adapted to the climatic conditions and has spread rapidly across the island, putting a dent in terrestrial invertebrate communities and seabird populations alike. The island’s ecosystem has never shared its evolutionary history with mice; thus their predator impact cannot be counteracted because the indigenous species have not developed defence mechanisms to cope with being attacked. For seabirds, this is most severe for chicks, which are more vulnerable to attacks than adults.
The mice have a varied diet that includes invertebrates and grass seeds to capitalise on a range of available sources of food. The omnivorous nature of their diet allows the mice to cope with seasonal changes in invertebrate food supply by shifting to alternative prey such as seabird chicks. Given that most seabirds at Marion breed in summer, mice predation can be more apparent at this time. However, mice attacks also occur in winter-breeding species, including the burrow-nesting Great-winged Petrel Pterodroma macroptera. In fact, mouse attacks of seabirds are more prevalent in winter due to reduced quantities of other food. Marion Island can be extremely cold and windy, so underground burrows offer better insulation from the harsh conditions. Studies (e.g. Dilley et al. 2017) have shown the direct detrimental impacts of mice on burrow-nesting petrels, with significant fatalities of chicks, leading to reductions in breeding success and a stagnant population recovery rate following the eradication of the feral Cats Felis catus in the 1990s. Here, we report on the predation impact of mice on the chicks of Great-winged Petrels through observational accounts and burrowscope inspections.
Every year Oceans and Coasts Ornithological Field Assistants of the Department of Forestry, Fisheries and the Environment (DFFE) monitor the contents of a number of breeding burrows belonging to the winter-breeding Great-winged Petrel. In 2022, the overwintering team (Andile Mdluli and Samuel Peta) took over the reins of this long-term monitoring project to understand the breeding ecology of the Great-winged Petrel and the impacts of mice on their breeding success. Since the commencement of monitoring this year in mid-May a total of 21 mice has been spotted in or close to the monitoring colony at Nellie Humps. However, these opportunistic mouse observations offer only anecdotal evidence of their impact on the breeding success of the Great-winged Petrel. To add to these observations, this year we deployed two trail cameras at the entrances of two occupied burrows. The cameras were set in a high-sensitive mode to detect mice entering or leaving the burrows.
A total of 6500 images from 15 August and 19 September 2022 was analysed for the presence of mice. At both burrows, mice were seen on the camera footage multiple times, with most detections occurring during the day and only a few occurrences at night. Unfortunately, the nest which had the most mouse activity failed (see burrowscope image below). The increased mouse activity during the day revealed an interesting pattern for both nests. We observed that chicks are often left alone with camera footage capturing no movements of adult birds during the day. Mice appeared to enter, exit or linger around burrows most frequently between 11h00 and 17h00. Although Great-winged Petrels are nocturnally active ashore, we found that breeding adults often spend time out at sea at night. For both breeding attempts we observed that adults returning to their burrows after 18h30, leaving shortly thereafter and then returning again close to midnight; they leave for a second time between 03h00-05h00 (but noting the parent birds were not individually distinguishable). These times are crucial in understanding the impacts of mice, highlighting that Great-winged Petrel chicks are most vulnerable to attack during daylight when parent birds are absent.
Mice do occasionally visit burrows at night, as shown by the image analysis. However, on two occasions, adult Great-winged Petrels were observed to chase the mice out of their burrows. it remains to be seen if the only 12 surviving chicks out of the original 72 will successfully fledge.
To curb the gruesome carnage inflicted by House Mice on Great-winged Petrels at Marion Island, the eradication of mice is a necessary action. The results presented here provide further evidence of the dire situation for seabirds nesting on Marion Island and the urgent need for a mouse eradication intervention.
Dilley, B.J., Schoombie, S., Stevens, K., Davies, D., Perold, V., Osborne, A., Schoombie, J., Brink, C.W., Carpenter-Kling, T. & Ryan, P.G. 2018. Mouse predation affects breeding success of burrow-nesting petrels at sub-Antarctic Marion Island. Antarctic Science 30: 93-104.
Samuel Peta, Andile Mdluli, Makhudu Masotla & Azwianewi Makhado, Department of Forestry, Fisheries and the Environment, Cape Town, South Africa, 02 February 2023
The Mouse-Free Marion Project is a registered non-profit company (No. 2020/922433/08) in South Africa, established to eradicate the invasive albatross-killing mice on Marion Island in the Southern Ocean. The project was initiated by BirdLife South Africa and the South African Department of Forestry, Fisheries and the Environment. Upon successful completion, the project will restore the critical breeding habitat of over two million seabirds, many globally threatened, and improve the island’s resilience to a warming climate. For more information or to support the project please visit mousefreemarion.org.
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