Sexual size dimorphism and sex determination in Blacksmith Lapwing Vanellus armatus (Burchell, 1822) (Charadriiformes: Charadriidae)

Abstract

Sexing monomorphic birds by DNA and morphometrics allows researchers to study behavioural differences between the sexes. We aimed to verify the utility of the carpal spur to sex the monomorphic Blacksmith Lapwing or to determine a more suitable method. The Blacksmith Lapwing is a widespread resident of central and southern Africa. We used biometrics and carpal spur features of 116 Blacksmith Lapwings we ringed at five South African sites in 2008–2018 that were DNA-sexed from blood samples. Males were on average larger than females in most morphometrics, but all these measurements largely overlapped. Spur length was the most sexually dimorphic trait, then body mass and tarsus length. Male lapwings use their wing spurs in combat, thus the spurs’ length and sharpness, and body size, are likely subject to intrasexual selection. The male’s longer tarsus may be related to his role in scraping out the ground nest. Our backward stepwise analysis produced the discriminant function D1 = 0.366 tarsus length - 27.541 to determine this species’ sex, which correctly sexed 66% of our sample. If we allowed a 5% misclassification rate then 70% of individuals remained unsexed. Thus we recommend DNA analysis for studies that must accurately sex all Blacksmith Lapwings in a sample. All of the DNA-sexed adult males had sharp ivory tipped spurs but 36% had a spur shorter than 13 mm, proposed previously as the cut-off value between sexes. We suggest that Blacksmith Lapwings with sharp ivory-tipped spurs of 14 mm or longer can be safely sexed as males to reduce the need for expensive DNA tests.