Diet of the introduced Gough Moorhen Gallinula comeri

107 Citation: Bond AL & McClelland GTW. 2021. Diet of the introduced Gough Moorhen Gallinula comeri on Tristan da Cunha. Ornis Svecica 31: 107–112. https://doi.org/10.34080/os.v31.23476. Copyright: © 2021 the author(s). This is an open access article distributed under the CC BY 4.0 license, which allows unrestricted use and redistribution, provided that the original author(s) and source are credited. S HOR T COMMUNIC AT ION

Tristan da Cunha (37°7'S, 12°18'W) and its associated islands (Nightingale, Inaccessible, and Gough islands; Figure 1) are among the most remote in the world, and consequently have a high number of endemic species (Wace & Holdgate 1958, Holdgate 1965, Wace & Dick son 1965. Tristan is home to c. 250 residents, and is nearly circular, covering 96 km2 with the conical Queen Mary's Peak rising 2060 m. Of all the islands in the group, Tristan is geologically the most recent and only 0.2-0.5 million years old (Ryan 2007). Since human set tlement in the early 19 th century, an increasing number of introduced and invasive species have arrived on the much of the island including the settlement plain (Rich ardson 1984). There was considerable debate about whether G. nesiotis had persisted or indeed whether G. nesiotis and G. comeri were the same species (Broek huy sen & Macnae 1949, Eber 1961. Molecular evidence now indicates that they are indeed separate taxa and that G. nesiotis is extinct (Groenenberg et al. 2008).
The ecology of the Gough Moorhen on Tristan, however, has received scant attention with the only de islands, both intentionally and accidentally (Wace 1967, 1969, Wace & Holdgate 1976, Jones et al. 2003, Ryan 2007, where they have had devastating consequences (Angel & Cooper 2006, Ryan et al. 2006, Wanless et al. 2009, Caravaggi et al. 2019, Dilley et al. 2020. In the South Atlantic Ocean, flightless moorhens Gallinula spp. were found on both Tristan da Cunha and Gough Island. On Tristan, the Tristan Moorhen Gallinula nesiotis went extinct in the late 19 th century, likely as a result of pre dation by introduced cats Felis catus (eradicated in the early 1970s) and human persecution (Sperling 1872, Richardson 1984, Bond et al. 2019. Only two speci mens exist, both in the Natural History Museum, Tring (Knox & Walters 1994, Bond et al. 2019. On Gough, the Gough Moor hen Gallinula comeri per sists, and 2,000-3,000 pairs are found throughout the island (Watkins & Furness 1986). In 1956, six Gough Moorhens were introduced to Tristan at Sandy Point on the east coast, and they are now present through  Richardson (1984) found seeds, vegetation and gravel, but also eggshells and a cephalopod beak in 1973 and 1974. The specimens were deposited in the Natural History Museum, Tring (NHMUK 1984.7.1-4), but contain no further details of diet or stomach contents. We examined the stomachs of four Gough Moor hens on Tristan da Cunha in the summer of 2014/2015.
Birds were killed accidentally during rodent trapping at Pigbite (n = 2) and Big Point Road (n = 2) at 270-350 m elevation. Birds were sexed either by dissection or using measurements (Parker et al. 2012). The complete pro ventriculus and gizzard were dissected out and flushed with tap water. The contents were examined under a 10-40 × dissecting microscope, and prey identified to the nearest 5 % by volume.
Of the four examined, the stomach of one was emp ty aside from six small bones, three of which were of house mouse tail vertebrae. The remaining three had consumed predominantly vegetation, likely one of the many endemic or introduced grasses on Tristan (Ryan 2007). Two had also smaller proportions of earthworms and spiders. Two birds had also consumed anthropo genic debris, including metal foil and glass (Table 1). support during our stay. Comments from anonymous reviewers improved this manuscript. There are six recorded species of earthworm (Oligo chaeta: Lumbricidae) on Tristan da Cunha, all nonnative and with a palearctic distribution (Reyn olds & Hänel 2005). The arachnid fauna has not been thoroughly studied, but many islands in the Southern Ocean have endemic species or those associated with South America or the neotropics (Pugh 2004).

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That moorhens scavenged or preyed on introduced rodents directly is not surprising, as they do the same on Gough Island where house mice are introduced; they also prey on seabird eggs and chicks (Watkins & Furness 1986, Wanless & Wilson 2007. Moorhens not only scavenge dead rodents, but also actively pursue and prey upon them (Wanless & Wilson 2007). If introduced rodents are ever eradicated from Tristan (Dawson et al. 2015, Holmes et al. 2019, and the moor hen population is to remain, a captive population will be required, as was recommended for Gough Island (Dagleish et al. 2017).
The occurrence of debris (glass and foil) in two of the stomachs is also expected. Waste on Tristan is managed by incineration at a site on the 1961 Volcano east of Edinburgh of the Seven Seas, and incomplete burning or escape from waste management streams is common. In the 1970s, one moorhen had ingested gravel (Richardson 1984), presumably as gastroliths to aid in digestion (Downs et al. 2019). This is the first occurrence of recorded debris ingested by terrestrial birds on Tristan da Cunha, where there is also a sig nificant influx of plastics from international shipping (Ryan et al. 2019). Moorhens are widespread on Tristan, and though uncommon along the coast, can forage on beaches, and likely have territories of c. 5,000 m2 (Wat kins & Furness 1986).
Together, these results confirm that introduced Gough Moorhens on Tristan have similar diets to those on Gough, namely a generalist, but primarily herbi vorous one with opportunistic scavenging and pos sibly direct predation of introduced rodents. Broader ecosystem restoration on Tristan must account for the potential impact on, and of, Gough Moorhens.