928 F. Zino et al.


weight. The chicks fledge in late October, c. 100 days after hatching (Zino et al., 1987). The island had harboured populations of two alien inva-


sive mammals (Howald et al., 2015), the house mouse Mus musculus and the European rabbit Oryctolagus cuniculus. Depredations inflicted on nesting seabirds and the floras of oceanic islands by these species have been described (Courchamp et al., 2003). A successful campaign of eradica- tion was conducted against these two species by staff of the Parque Natural da Madeira in the latter part of 2002 and early 2003 (Oliveira et al., 2010). Baits containing 50 ppm brodifacoum, a potent anticoagulant rodenticide, were applied from c. 17,000 baiting points established on a 12.5 × 12.5 m grid across the entire island, where the terrain permitted. Bait was also applied by hand on steep slopes and cliffs where bait stations could not be placed. Preliminary studies documented the beneficial effect of this action on invertebrate and plant communities and on populations of Berthelot’s pipit Anthus berthelotti berthelotti and two rep- tiles, T. dugesii selvagensis and Tarentola bischoffi (Oliveira et al., 2010). However, an objective of the eradication pro- gramme was also the protection of the internationally important breeding populations of five pelagic seabirds: Cory’s shearwater Calonectris borealis, Bulwer’s petrel Bulweria bulwerii, Barolo (or Macaronesian) shearwater Puffinus baroli (also referred to as Puffinus iherminieri or Puffinus iherminieri baroli), white-faced storm-petrel Pelagodroma marina hypoleuca and band-rumped storm- petrel Hydrobates castro. Of these, Cory’s shearwater is the most amenable to study because the terrain and locations of nests of the other species in unstable sandy substrates make population monitoring more difficult (Granadeiro et al., 2006). The island’s Cory’s shearwaters, however, build nests in accessible areas and are tractable to handling during laying and incubation without risk of nest desertion (Zino, 1971; Zino et al., 1987). A long-term examination of the breeding biology of


Cory’s shearwater on Selvagem Grande began in 1971 with the establishment of two quadrats (named ABCD and EFGH; Zino, 1971; Zino et al., 2008), and nests have been monitored ever since. This study has provided an opportu- nity to observe changes in the breeding biology of Cory’s shearwater of Selvagem Grande over a period of almost 50 years. Monitoring of the benefits of island renovation pro- grammes is generally lacking because logistical difficulties may prevent return to the islands concerned and scarce resources are preferably spent on extending programmes to additional islands (Brooke et al., 2007). However, the study of Cory’s shearwater on Selvagem Grande has per- mitted an assessment of the beneficial impacts of the erad- ication of invasive mammals on this species. A preliminary assessment, published 4 years after the


eradications, showed an increase of 23–47% in the fledging rate of Cory’s shearwater in the two quadrats (Zino et al.,


2008). The eradication began around the time of hatching in 2002 and fledging rates began to increase in that year. This suggests that the effects of the invasive mammals were on hatchlings and developing chicks, rather than by disturb- ance of pairing, courtship, nest-building and egg-laying or direct predation of eggs. It was, and still is, impossible to knowwhich biological processes had been causing depression of fledging rates prior to the eradication, but it seems likely it was predation of newly-hatched chicks by house mice (Cuthbert & Hilton, 2004). Study of the breeding Cory’sshearwaterinthe twoquad-


rats has continued since the assessment of Zino et al. (2008), and there are therefore now 19 years of data since the removal of rabbits and house mice in 2002.Hereweprovidethese add- itional data on fledging rates and confirmation of the bene- ficial impacts of the removal of rabbits and mice on this population of Cory’s shearwater and, probably, on the other, smaller and thus more vulnerable seabirds on the island.


Study area


The 270 ha SelvagemGrande is the largest island in an archi- pelago in the north-eastern Atlantic, between Madeira, 300 km to the north, and the Canaries, 150 km to the south. The geology is volcanic, with a semi-arid ecosystem (Mata et al., 2010). Approximately 150 ha of the island is a sandy plateau used for nesting by burrowing seabirds, with other areas comprising boulder-strewn slopes, with gradi- ents from gentle to precipitous, and there are sea cliffs in the north rising to c. 100 m along 25% of the coastline. The island, and neighbouring Selvagem Pequena and


Ilhéu de Fora, are protected under Portuguese Law as a Strict Nature Reserve, and by international regulations con- cerning their bird species (Grimmett & Jones, 1989;Costa et al., 2003;European Union, 2009), and administered by the Instituto das Florestas e Conservação da Natureza. Selvagem Grande is an important breeding location for five pelagic seabird Species of European Conservation Concern (Oliveira & Menezes, 2004; BirdLife International, 2017). The two study quadrats were established on opposite


sides of the island (Fig. 1). Quadrat ABCD is c. 16 × 140 m at Baía das Pardelas in the east, exposed to the north-east trade winds. Quadrat EFGH is c. 15 × 100 m, in the south- west below the lighthouse on Pico da Atalaia, and relatively sheltered. Both quadrats are in broken and rocky terrain, on slopes of 15–35%.


Methods


Transport to the island was provided by Portuguese Naval patrol vessels but because of weather conditions and oper- ational necessities, visits to the island were sometimes ir- regular and of variable duration. Nevertheless, in most


Oryx, 2022, 56(6), 927–934 © The Author(s), 2021. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605321000661


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