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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 123, No. 4. December 2011 
defense of territories) when morphometric char¬ 
acteristics were measured. A similar pattern of a 
putative male-biased sex ratio was reported by 
others at the same stage of the breeding cycle 
(Maichant and Higgins 1990). The strong statistical 
significance of the discriminant functions, the 
consistency of the three DFAs in selection of 
discriminating variables (MBL and HL). and the 
high proportion (95%) of individuals that were 
correctly classified, despite the relatively low 
number of females, suggests our results represent 
a real biological pattern. We acknowledge our 
findings could be biased if different-sized males 
and females returned to the colony at different 
times of the breeding cycle (e.g., smaller females 
visited the colony earlier than did larger individ¬ 
uals). However, we know of no evidence suggest¬ 
ing such a pattern. 
We detected clear morphometric differences 
between females and males. On average, the body 
mass ol males was 6% larger than females; male 
procellariiform seabirds are commonly larger than 
females (Brooke 2004). Our DFAs demonstrated 
characteristics of the head and bill (HL and MBD) 
were the best discriminators of gender, a finding 
reported in other studies of proceltariids (Gcno- 
vart et al. 2003, Guicking et al. 2004. O’Dwyer et 
al. 2006, Bourgeois et al. 2007, Thalmann et al. 
2007, Navarro et al. 2009). Bill sizes of the 
Westland Petrel were consistently strong predic¬ 
tors for classifying gender, in parallel with the 
statistically similar findings reported in prior work 
using DFA in two congeners, White-chinned 
Petrel (P. uequmoctialis ) and Grey Petrel ( P 
cine tea) (Brooke 1986, Ryan 1999). 
Differences in morphometric characteristics may 
have consequences for how females and males vary 
m foraging behavior (Kato et al. 2000. Phillips et al. 
2004). This may be particularly true for differences 
in head and bill characteristics. For example, the 
larger bill of male Southern Giant Petrels {Macro- 
nectes gigame us) may make them more efficient 
than females when competing for access to 
carcasses {Gonzalez-Sol (s and Croxall 2005). We 
found distinct differences in head and bill charac¬ 
teristics between female and male Westland Petrels' 
however, at present it is unclear whether females 
and males forage differently. These morphological 
inferences also may be due to sexual selection and/ 
or reproductive role division, potentially serving as 
nan h c r a S d S,g r:' ,01 ' miUe and/or mainte- 
Temele X£Z °‘ <H « lrick antI 
es Navarro et al. 2009). 
Our study is the first to develop and use DFA to 
discriminate female and male Westland Petrels 
We suggest the canonical discriminant and 
classification functions we derived be validated 
with additional samples. We also recommend that 
morphometric measurements be taken during 
different times of the breeding season and 
between years, as some measurements may van 
over these scales (Phillips and Furness 1997) 
Standard molecular and histological methods for 
classification of gender of birds, such as DNA 
hybridization. Polymerase Chain Reaction (PCRi 
and cloacal examination, while preferred, also 
have several limitations (Daniel et al. 2007). For 
example, use of molecular techniques may not be 
practical in the field when knowledge of gender of 
study subjects is immediately required. Similarly, 
accuracy of cloacal examination may be limited 
outside of the incubation phase (O’Dwyer et al. 
2006). Our study shows promise in use of select 
morphometric characteristics for predicting gen¬ 
der of Westland Petrels under field conditions. 
ACKNOWLEDGMENTS 
We thank the New Zealand Department of Conservation 
stutl from the Punakaiki Field Centre, particularly G. C. 
Wood, the West Coast Conservancy, and the National 
Banding Office for facilitating access io conduct this 
research, assistance, and support. We thank Grant Ballard. 
M. D. Brooke. S. M. H. Isrnar. C. D. MiJlar. M. J. Rayner. 
and many other colleagues for discussions of this topic 
Hus research was funded by the University of Auckland 
through the Faculty of Science Professional Development 
Fund and School of Biological Sciences performance-based 
research fund, and the Auckland Museum T J. L was 
supported hy a University of Auckland Doctoral Scholar¬ 
ship. 
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