96 The Use of Morphometric Measurements to Sex Yellow-eyed Penguins ALVIN N. SETIAWAN 1 , JOHN T. DARBY 1 AND DAVID M. LAMBERT 2 1 Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand 2 Allan Wilson Centre for Molecular Ecology and Evolution, Institute of Molecular BioSciences, Massey University Private Bag 102 904, Albany, Auckland, New Zealand Abstract.— The Yellow-eyed Penguin (Megadyptes antipodes) is monomorphic but exhibits subtle sexual morpho- metric dimorphism. Information on the sexes of these birds is needed for informed management of the species and the construction of accurate population models. Using DNA analysis and Discriminant Function Analysis on head and foot measurements, we tested whether adults and fledglings of Yellow-eyed Penguin can be sexed using mor- phometry. We found that head and foot measurements can be used to correctly sex up to 93% of adults. Using only foot length, 88% of fledglings can be sexed accurately. As age until fledging was found to have an effect on mor- phometric variables, sexing should be conducted as synchronized as possible when chicks are over 90 days of age. We recommend that error rates inherent in sexing fledglings be taken into account when publishing sex-ratio data and subsequent analyses, particularly when age of fledglings could not be determined. The use of whole-skull mea- surements for sexing may be useful in other species in which bill measurements between the sexes are only slightly dimorphic or contains large degrees of overlap. Received 10 April 2003, accepted 12 December 2003. Keywords.—Yellow-eyed Penguins, morphometrics, fledglings, sex identification, discriminant function analy- sis, sexual dimorphism. Waterbirds 27(1): 96-101, 2004 The Yellow-eyed Penguin (Megadyptes an- tipodes) is an endangered species endemic to the southeast coast of the South Island and Subantarctic islands of New Zealand (March- ant and Higgins 1990). Due to its endan- gered status, the species is intensively managed on the mainland. As such, infor- mation on the sexes of these birds is vital to provide accurate demographic data and con- struction of population models. Further- more, studies on the behavior (Seddon and Darby 1990; Van Heezik and Davis 1990; Schuster and Darby 2000), ecology (e.g., Darby and Seddon 1990; Massaro et al. 2002) and physiology (Farner 1957; Cockrem and Seddon 1994) often require the sex of indi- viduals to be known. The sexing criteria used in the past and as suggested by Richdale (1951) are impractical, involving close obser- vations during the breeding season, requir- ing detailed observation over time. A method to easily and quickly sex individuals in the field is needed, especially for use in ar- eas where close observation is impractical (e.g., offshore islands). In many seabirds, subtle morphometric dimorphism can be used for sex assignment, e.g., Shy Albatross (Diomedea cauta; Hedd et al. 1998), gulls (Coulson et al. 1983; Jodice et al. 2000), Cory’s Shearwater (Calonectris dio- medea; Lo Valvo 2001) and Balearic Shear- water (Puffinus mauretanicus; Genovart et al. 2003). Measurements made following the post-mortem dissection of Yellow-eyed Pen- guins showed that both the head (head and bill length) and foot of males were on aver- age 6 mm longer than those of females (Dar- by and Seddon 1990). This dimorphism may be useful for sex identification. Yellow-eyed Penguins fledglings near the end of their growing period may similarly ex- hibit detectable dimorphism that could be used for sex identification, as in the Magel- lanic Penguin (Spheniscus magellanicus; Sco- laro 1987) and Shy Albatross (Hedd et al. 1998). Van Heezik (1990) showed that Yel- low-eyed Penguin fledglings (two weeks pri- or to fledging) had attained adult head and foot sizes. The availability of highly reliable DNA- based techniques for sex identification of many bird species in recent years provides the opportunity to test if Yellow-eyed Pen- guins can be reliably sexed using morpho- metric measurements. The aims of this study were to determine if morphometric mea- surements could be used to reliably sex Yel- low-eyed Penguin adults and fledglings.