Can eyeflecks be used to sex African Black Oystercatchers Haematopus moquini in the field?
Résumé
Morphometric differences between males and females are a common feature among oystercatcher species (Hockey 1996), although breeding partners often appear similar when observed from a distance. Combinations of different biometric parameters such as bill size and shape, body mass, wing and tarsus lengths have been used to discriminate males and females in the field in European Oystercatchers Haematopus ostralegus (Zwarts et al. 1996), American Black Oystercatchers Haematopus bachmani (Guzzetti et al. 2008) and the three oystercatcher species present in New Zealand (Baker 1973). Sexual dimorphism also occurs in the African Black Oystercatcher Haematopus moquini, an endemic species living on the coasts of Namibia and South Africa. In this species, females tend to have longer and sharper bills than males (Hockey 1981, Hockey 2005). However, sex determination based solely on this parameter can be inaccurate because the ranges of bill size between males and females overlap. Variations in bill morphology between sites and/or regions may also occur in relation to feeding habits and prey assemblages (Hulscher 1996). Another way to sex oystercatchers is checking for cloacal distension in females during the period immediately after egg laying. Although molecular markers remain the most reliable tool for sexing non-ratite birds (Fridolfsson and Ellegren 1999), these techniques require capturing the birds, are relatively expensive and the 25 between the CHD1W and CHD1Z introns located on the sex chromosomes (Fridolfsson and Ellegren 1999).