Grey-faced petrel (Pterodroma gouldi) productivity unaffected by kiore (Pacific rats, Rattus exulans) on a New Zealand offshore island

Figures & data

Figure 1 Map of Ririwha (Stephenson Island, Mahinepua), New Zealand, showing location of plots for monitoring grey-faced petrels (Pterodroma gouldi). Kiore (Rattus exulans) were controlled by kill-trapping in the 2010 petrel breeding season at plots T1 and T2 and monitored by index trapping at the same plots in 2011 and 2012. Inset shows the location of Ririwha relative to the northeastern coast of New Zealand.

Figure 2 Timeline and capture rates of kiore (Rattus exulans) at two plots (T1—129 traps, ▴; T2—155 traps, ●) on Ririwha (Stephenson Island, Mahinepua), northeastern New Zealand. Rates are expressed as captures per 100 corrected trap nights (C/100 CTN) and are shown for the initial 41-night removal programme and for subsequent three-night index trapping. Widespread poisoning of rats also took place in June and July 2011(light grey shading). Grey-faced petrel (Pterodroma gouldi) nesting periods are indicated by darker grey horizontal bars. Note: no rats were captured at either site during the October 2010 index trapping.

Figure 3 Cumulative kiore (Rattus exulans) captures over 41 days at two removal plots on Ririwha (Stephenson Island, Mahinepua) off northeastern New Zealand, July 2010.

Table 1 Numbers of burrow entrances and rates of burrow occupancy by breeding grey-faced petrels on Ririwha (Stephenson Island, Mahinepua), New Zealand. Plots are 40 × 40 m and were monitored from 2010 to 2012. Estimated daily nest survival rates are for all plots combined. Period survival is over the full 172-day combined egg and chick period. Values in parentheses are 95% confidence intervals.

Plot	Number of burrow entrances	Proportion occupied
		2010	2011	2012
C1	47–51	0.549	0.532	0.469
C2	44–46	0.682	0.727	0.739
C3	22–24	0.542	0.455	0.458
C4	83–87	0.713	0.590	0.644
C5	45–50	0.733	0.625	0.560
C6	29–38	0.474	0.600	0.552
T1	60–62	0.468	0.467	0.517
T2	32–36	0.639	0.636	0.594
Daily nest survival rate		0.9925 (0.9918–0.9932)	0.9933 (0.9926–0.9940)	0.9923 (0.9916–0.9929)
Period nest survival		0.275 (0.241–0.309)	0.317 (0.281–0.354)	0.264 (0.233–0.295)

Table 2 Documented impacts of kiore on a range of procellariiform seabirds relative to seabird adult body mass. Adult body mass estimates are mean values except, where a range was given, the median.

Species	Adult mass (g)	Nesting habit	Impact	Source
Diving petrel	130	Burrow	Indirect observation of egg predation; marked range expansion over 7 years following rat removal	Thoresen (1967); Towns et al. (2006)
Pycroft's petrel	130	Burrow	Productivity on two islands approx. double when kiore removed	Pierce (2002)
Bonin petrel	180	Burrow	Egg/chick predation inferred from negligible fledging rates	Kepler (1967)
Cook's petrel	190	Burrow	Predation of eggs and chicks	Imber et al. (2003); Rayner et al. (2007)
Little (allied) shearwater	240	Burrow	Egg predation; increased productivity when kiore removed	Booth et al. (1996); Pierce (2002).
Herald petrel	277	Surface	Predation, primarily of young chicks at nests	Brooke (1995)
Henderson petrel	277	Surface	Predation, primarily of young chicks at nests	Brooke (1995)
Murphy's petrel	435	Surface	Predation, primarily of young chicks at nests, but varied between islands (lower where rats smaller and less abundant)	Brooke (1995)
Kermadec petrel	510	Surface	Predation, primarily of young chicks at nests	Brooke (1995)
Grey-faced petrel	550	Burrow	No effect on nest survival	This study
			No observable change in abundance 10 years after removal of kiore	Towns et al. (2006)
Laysan albatross	3090	Surface	Observation of attacks by multiple rats	Kepler (1967)

Sources: Warham (1977); Marchant & Higgins (1990); Spear et al. (1992); Brooke (1995); Spear & Ainley (1998); Heather & Robertson (2005).

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