Breeding of Cory’s shearwater 933
therefore relied more on Cory’s chicks for food, or (3) be- cause the environment of Selvagem Grande is arid, liquids available as chicks emerge from egg membranes may have stimulated mice to attack chicks. The Madeiran wall lizard also predates Cory’s shearwater hatchlings on Selvagem Grande (Matias et al., 2009), andwe also observed this species feeding on the carcases of chicks, more so after the mammal eradications, and occasionally attacking live chicks. Matias et al. (2009) studied 78 shear- water nests in 2006 and 153 in 2007, and recorded fledging rates of 71 and 73%, respectively. Predation by lizards was responsible for 18.2–19.5% of nest failures and estimated to have caused a reduction in nest productivity of 5%. Oliveira et al. (2010) showed that the island’s population of the gecko T. bischoffi increased on Selvagem Grande as a result of the mammal eradications and it seems likely that the Madeiran wall lizard also benefited, probably by the removal of mice that may prey upon lizard eggs and young. Matias et al. (2009) concluded that the Cory’s shear- water population could tolerate lizard predation, but their nests were more productive than those in our larger and long-term study of Cory’s shearwater, even after the removal programme (Table 2). Thus, the increase in the fledging rates of Cory’s shearwater apparently achieved by the re- moval of mice from Selvagem Grande may be partly offset by increased predation by lizards, whose population was itself released from mouse predation. Since the culls of Cory’sshearwaters(the lastin 1967)
and the illegal taking of adult and chicks thereafter, cul- minating in the events of 1976, population recovery has gone through a number of stages. Mougin et al. (2000) recorded population growth of c. 30% per annum during 1980–1985, attributing this to the recruitment of subadults that had been in their first decade, and therefore at sea, during the 1976 slaughter. During 1985–1995,Mougin et al. (1996) estimated a slower population annual growth rate of c. 5%. A census of breeding Cory’sshearwaterson Selvagem Grande in 2005 (Granadeiro et al., 2006)esti- mated a population of 29,540 breeding pairs, giving an average annual population growth rate of 4.6%since the earlier estimate of 18,000 breeding pairs (Mougin et al., 1996). Granadeiro et al. (2006) estimated it would take the population 35 years to recover to its presumed level prior to the culls (c. 140,000 breeding pairs). It is likely that, so long as there are no new and unforeseen threats to Cory’s shearwaters, the rate of population increase on Selvagem Grande will accelerate as a result of the signifi- cant increases in fledging rates that we report. During 2009–2020, for which our data permit the esti-
mation of nesting success, it is apparent that c. 35%of eggs laid in the two quadrats failed to produce a fledged chick (Table 2). This is similar to an estimate of egg loss dur- ing incubation of 32%(Mougin et al., 1996), obtained before mice and rabbits were removed. Granadeiro et al. (2006)
recorded a loss of 25% of clutches during the 2005 breeding season, which was 3 years after eradication. The 12-year span of our data provides information on the annual variation in nest failure (Table 2). The highest was in quadrat EFGH in 2014, when 46%of nesting attempts failed, whereas in quad- ratABCDin 2009 the failure rate was only 20%. Annual var- iations in nest failure are potentially caused by a range of factors, including: (1) predation by theMadeiran wall lizard, either a result of annual variation in the lizard population or in the availability of alternative food and water, (2) food availability for the adults at sea, and consequently feeding rates for chicks, and (3) adverse weather conditions on the island during the nesting period (heavy rain may occur in September and October, and juveniles are sometimes found either washed off their nests or interred in burrows by soil deposited by flood water). However, we do not know the relative importance of these three factors, nor even if they are the most important. With a head–tail length of 60 cm and amean female body
weight of c. 750 g, and c. 900 g for males, Cory’sshearwateris the largest of the five seabirds nesting on Selvagem Grande and therefore may have been the species least affected by housemouse predation. It seems reasonable to assume, there- fore, that emergence and fledging of smaller species, such as the band-rumped and white-faced storm-petrels, would have improved at a rate similar to that of Cory’sshearwater. However, this remains speculative because there are no de- tailed studies of the other four species and potential predation by lizards is a confounding factor.We believe our long-term study provides encouragement to those involved in the man- agement of alien invasive mammals elsewhere and incentive to those who conduct eradication programmes to monitor the impacts of this essential conservation work.
Acknowledgements Wethank the Portuguese Navy for transporta- tion, the Parque Natural da Madeira (now Instituto das Florestas e Conservação da Natureza) for permission to work on the island; the wardens, particularly Jaques da Mata and Carlos Santos, for their as- sistance with field work; Centro de Estudos de Migrações e Proteção de Aves (Instituto da Conservação da Natureza e das Florestas, Lisbon) for providing bird rings; Elizabeth A., wife of FZ, and their son Alexander and daughter Francesca, and a number of volunteers for help during visits and fieldwork; the University of Reading for funding the publi- cation of the article; and two reviewers for their helpful and construc- tive comments.
Author contributions Fieldwork: FZ, the late P.A. Zino, MB; data analysis: the late Mike Hounsome,AB; writing: AB; revision: all authors.
Conflict of interest None.
Ethical standards During the long duration of the study the ethical standards applicable varied. Permission to handle and ringCory’sshear- waters on Selvagem Grande was initially granted by the Parque Natural da Madeira and latterly by the Instituto das Florestas e Conservação da Natureza. Ringing protocols applied were those provided by Centro de Estudos deMigrações e Proteção deAves, the national ringing authority
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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