932 F. Zino et al.


TABLE 2 The numbers of nesting attempts and the per cent of breeding attempts that were successful of Cory’s shearwaters in the two quadrats (ABCE, EFGH) on the island of Selvagem Grande during 2009–2020. A nesting attempt was considered to have occurred when birds had paired and an egg was laid. Per cent of attempts successful is derived from the number of nesting attempts and the number of fledged chicks (Table 1). It is only during 2009–2020 that sufficient visits were made to the island to obtain these data (see text for further details).


ABCD


Year 2009


2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020


Mean ± SD


Number of nesting attempts


167 217 217 183 212 224 237 264 269 264 277 264


232.92 ± 35.70


% attempts successful


80.24 56.22 57.14 78.14 58.96 57.59 67.09 59.47 72.86 70.45 64.62 67.42


65.85 ± 8.31 EFGH


Number of nesting attempts


146 177 183 156 178 188 192 221 224 229 242 214


195.83 ± 30.11


developed for the management of alien invasive vertebrates and the benefits of eradication programmes, challenged by some on ethical grounds (Howald et al., 2015), can be justi- fied. The 39-year data set (1982–2020) for the fledging of Cory’s shearwater chicks on SelvagemGrande has provided an unparalleled opportunity to investigate the biological benefits of mammal eradication on an island. The beneficial effect of the eradications on Cory’sshear-


waterisclear fromthedatapresented here (Table 1, Figs 1&2). However, because two mammal species, the European rab- bit and the house mouse, were removed from the island si- multaneously we are unable to separate the impacts of the two species prior to their eradication. The best evidence that housemice rather than rabbits had the greatest influence on fledging rates comes from the observation that increased nest productivity occurred in 2002,even beforeeradication had been completed.The first bait application took place dur- ing 8–20 August 2002 (Oliveira et al., 2010), and would have removed the majority of both mammal populations quickly. This was the peak of hatchling emergence and we conclude that it was the adverse impacts of house mice on hatchlings that depressed fledging rates before eradication. Such direct predation is a characteristic of housemouse rather than rabbit behaviour (Courchamp et al., 2003;Cuthbert&Hilton, 2004) and research onother islands has elucidated the predatory be- haviour of house mice where they inhabit seabird colonies (Cuthbert, 2004; Cuthbert & Hilton, 2004; Wanless et al., 2007). Prior to eradication there were up to 300 house mice per ha on Selvagem Grande (Oliveira et al., 2010). Recording the numbers of nests in each quadrat (Table 1),


although either carried out or supervised by FZ, was poten- tially subject to some imprecision and stochasticity because


% attempts successful


72.60 62.15 64.48 73.72 57.87 49.47 65.10 61.54 70.09 63.76 57.44 62.62


63.40 ± 6.78 Combined


Number of nesting attempts


384 394 400 339 390 412 429 485 493 493 519 478


434.67 ± 16.39


% attempts successful


62.50 58.88 60.50 76.11 58.46 53.88 66.20 60.41 71.60 67.34 61.27 65.27


63.54 ± 6.13


finding and enumerating nests in the rough terrain requires diligence and skill, and also because of occasional uncer- tainty about whether nests at the boundaries of the quadrats fell within them. Therefore, the most reliable measure of the increase in breeding performance of Cory’s shearwater on Selvagem Grande after the eradication is the per cent of nest sites that successfully fledged young (Table 1). This par- ameter is not affected by stochasticity related to decisions about nest locations. The observed increase in Cory’s shear- water fledging rates was not uniform between the two quad- rats. In the 14 years of records before eradication, amean of only 37.0 nest sites per hundred fledged a chick in quadrat ABCD, and in quadrat EFGH this value was significantly higher at 45.3.Acount of all accessible nests across the island found that increases in the numbers of nests during 1985– 2005 were greater on the south and east coasts (6.1% per year), where EFGH is situated, than elsewhere (4.3%) (Granadeiro et al., 2006). This suggests recruitment was lower in some parts of the island, particularly in the north, because there were fewer nesting cavities and, therefore, poorer breeding conditions. However, fledging rates in our two quadrats, although different prior to mammal removal, did not differ significantly afterwards. We suggest that the alien invasive mammals, in particular house mice, may have played a role in the variation in fledging rates in differ- ent parts of the island prior to eradication. Potential causes of this could be: (1) house mouse abundance may have dif- fered, perhaps driven by the availability of food, water and harbourage, so there may have been more mice present in some areas, (2) less alternative food for house mice in some parts of the island, possibly affected by different mi- croclimates and soil substrates, and mice in some areas


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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