Breeding of Cory’s shearwater 929
FIG. 1 Selvagem Grande, with the positions of the quadrats ABCD and EFGH used for monitoring the fledging of Cory’s shearwater Calonectris borealis.
years we made at least two visits, the first during egg-laying (late May–mid June) to locate and enumerate nests within the quadrats, and the second in either September or October so that chicks could be ringed prior to fledging. Study visits to the island became more consistent after 1982, when the navy started regular visits, and we therefore present the data from that year onwards. From 2009 on- wards, we made sufficient visits to the island to obtain annual estimates of the per cent of nest failures by moni- toring nests throughout the breeding period. Principal data obtained from the quadrats annually were:
(1) total number of nest sites, (2) total number of chicks that fledged from the nests found, and (3) per cent of nest sites fromwhich a chick fledged (Table 1). Each nest was allocated a numeral that was marked nearby, and therefore nest sites were defined as any location in which eggs had been laid and previously enumerated (Table 1). All chicks were ringed. Analysis was conducted using data from 14 years prior
to eradication of the house mouse and European rabbit (1982–2001) and 19 following years (2002–2020)(Table 1). The main variable examined in the comparison of fledging before and after eradication was the per cent fledging rate. This was the per cent of all nests, both new nests found dur- ing the season in question and all nest sites previously occu- pied and enumerated, in a quadrat during that season that had a live chick on the final visit to the island. Some loss of chicks may occur after this visit and before fledging but, short of a catastrophic weather event, it is reasonable to assume the number is very small. There were no nesting attempts at some of the enumer-
ated nest sites. To accommodate this, and to derive a value for the per cent of nesting attempts that resulted in a fledged chick each season, a second variable was derived from the data using only those nests in which an egg was laid. These data, available for 12 years when more frequent visits made to the island (2009–2020), permitted assessment of the per cent of nests lost after pairing and laying.
Results
During 1982–2020, sufficient data for evaluation of the fledging rates of Cory’s shearwater on Selvagem Grande were available for 33 years; 14 years prior to the 2002 eradi- cation programme and 19 years after it. Table 1 gives the numbers of sites in the two quadrats where Cory’s shear- waters had made nests and numbers of chicks that fledged in 1982–2020. In quadrat ABCD, the mean annual per cent fledging after eradication (52.6 ± SD 6.4), was significantly greater than the mean before eradication (37.0 ± SD 4.8; t = 7.60,df = 31,P = 0.001; Table 1) and likewise in quadrat EFGH, mean fledging rate after eradication (53.4 ± SD 4.8) was significantly greater than before (45.3 ± SD 5.0; t = 4.68, df = 31,P = 0.001). Combined values for fledging in the quadrats (40.7 ± SD
3.9 fledglings per 100 nests before eradication and 52.9 ± SD 5.0 afterwards) were also highly significantly different (t = 7.50,df = 31,P = 0.001). The mean number of nesting sites recorded in the quadrats after the eradication (491.5 ± SD 69.1) was significantly greater than the numbers recorded annually before the eradication (381.6 ± SD 36.0; t = 5.41,df = 28,P,0.001; Fig. 2). Similarly, the mean number of chicks fledged after the removal of the alien in- vasive mammals (259.9 ± SD 44.5), was significantly greater than the mean number fledged before the event (155.1 ± SD 17.6; t = 8.31,df = 25,P,0.001; Fig. 3). Annual rates of in- crease for these parameters in the years after the eradication and relative to annual means prior to the event were 3.9%in numbers of nesting sites and 3.0% in numbers of fledged chicks. However, there was an increase in numbers of nests during pre-eradication years but not in numbers of fledged chicks (Table 1, Figs 2 & 3). All measured breeding parameters for Cory’s shear-
waters, both for quadrats separately and combined, showed highly significant increases after the removal of rabbits and house mice from Selvagem Grande. However, increases
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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