Feeding frequency and probability

The proportion of nights in which individual WFSP chicks were provisioned over the study period was lower than that recorded on Whero Island by a difference of 17.3% (n = 8) (Richdale, 1965). Differences in the methodology used to identify feeding events may account for this variation. Richdale (1965) considered a chick to have been fed when the morning weight was -0.5 g or more than the previous night’s weight. In this study, evening weights were not made and provisioning was considered to have occurred when overnight weight changed (from the previous morning) by less than 50% of mean overall weight loss of known unfed chicks (2.5 g), thus accounting for respiration and defecation (Rayner, et al., 2008). Young Procellariiform chicks are generally fed small meals frequently (Warham, 1990). This study quantified provisioning rates after chicks were approximately three weeks old. The difference in the time periods covered may account for the observed lower overall feeding rate of the current study. Provisioning was recorded as unknown when chicks went un-weighed for various reasons (rain/torpor) and/or feeding could not be determined. Such unknown events were excluded when calculating proportion of nights provisioned. The apparent provisioning probability observed in WFSP chicks (mean = 0.54) during this study was much lower than those reported for other storm petrel species. For example fork-tailed storm petrels have feeding probabilities ranging from 0.7 ± 0.6 /day(± SD, n=100) to 1.1 ± 0.7 /day (± SD, n=92) recorded over two consecutive years (Simons, 1981). Wilson’s storm petrel chicks were fed on 93% of nights, of which 68% were single feeds from one parent and 25% were meals from both parents (Quillfeldt & Peter, 2000).

In petrels, chicks are fed more frequently during the first half of chick development with the rates declining towards fledging (Gangloff & Wilson, 2004; Warham, 1990). In Pycroft’s petrel chicks (Pterodroma pycrofti) there are four distinct stages of declining provisioning probabilities, with the final three descents occurring during the final three weeks prior to fledging (Gangloff & Wilson, 2004). In this study the probability of WFSP chicks being provisioned on a given night relative to their age (DBF) showed no apparent pattern or correlation. Richdale (1965) showed the proportion of unfed night’s increases from 32% at 1–13 DBF to 40% and 68% at 8–5 DBF and 4–1 DBF respectively.

Procellariiform chicks, especially storm petrels, can withstand long fasting periods (Warham, 1990) and irregular chick provisioning is not uncommon among storm petrels (Ricklefs, Day,

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Huntington, & Williams, 1985). The growth of WFSP seems resilient to irregular feeding as the periods some focal chicks from this study went without provisioning (up to seven days) were much greater than expected from literature case studies. For example, southern WFSP chicks on Whero Island frequently went without provisioning for one to two days and only occasionally three to five days (Richdale, 1943b). Wilson’s storm petrels are fed on 93% of nights (Quillfeldt & Peter, 2000). During this study the frequency and duration at which WFSP chicks fasted did not affect their overall development or fledging morphology. As this study was over a single season, it cannot be determined if the rate of fasting is typical of this population. Variation in nestling periods is considered to be related to variation in parental provisioning (Mauck & Ricklefs, 2005; Warham, 1990) and the long fasting periods found may explain the longer chick rearing period (68 days) than on Whero Island (Richdale, 1965).

For the purpose of translocation, variation in fasting may affect the suitability of individuals for transfer selection. However, regardless of the fasting periods, when averaged out over time the quantity of food provisioned was uniform between most chicks (Figure 4.9). This suggests that the mass of food delivered compensates for periods of fasting (as proposed by Richdale, 1943a). Therefore, chicks which seem underweight may have fasted for a few days and re-weighing at a later date may help to increase number of those transferrable

The lower than expected provisioning rates and extended durations of fasting, of the 2011- 2012 WFSP breeding season may be due to seasonal conditions and it is difficult to

extrapolate from a single years results. Oceanic phenomena such as the warming effects of El Niño-Southern Oscillation (ENSO) influence marine resources and trophic flows by altering nutrients, plankton and other prey species abundance (Surman & Nicholson, 2009; Wolff, Ruiz, & Taylor, 2012). Seabirds are regarded as upper trophic level indicators of marine productivity (Surman & Nicholson, 2009) and the influence of ENSO can affect seabird breeding success, reproductive output and distribution (Jaksic, 2004; Surman & Nicholson, 2009; Wolff, et al., 2012). For example, dark-rumped petrel chicks (Pterodroma phaeopygia) show slower growth rates and latent fledging dates during ENSO (Cruz & Cruz, 1990). The peak weights achieved by chicks are smaller and attained later than chicks of non ENSO seasons (Cruz & Cruz, 1990). Although less well documented, La Niña-Southern Oscillation (LNSO) conditions also affect sea currents and food availability; altering the breeding

chronology of little blue penguins (Eudyptula minor) (Perriman, Houston, Steen, &

Johannesen, 2000) and reduced observations and of Oceanodroma,Pterodroma and Puffinus species (Bjorksteadt et al., 2011; Ribic, Ainley, & Spear, 1992). Mild to medium LNSO

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conditions were experienced in New Zealand over the summer of 2011-2012 following strong LNSO conditions of 2010-2011 (NIWA, 2010, 2011). Dunn (2012) suggest LNSO effects contributed to the observed stunted growth rates and low provisioning rates of grey-faced petrels breeding in the Hauraki Gulf during the 2011 breeding season. Similarly, WFSP provisioning biology may also be similarly affected by LNSO climatic conditions.