Criteria for sexing birds in studies of sexual dimorphism and sex-specific behavior

Emlen and Wrege (2004) reported interesting data on sexual dimorphism, sex-specific behaviors, sex ratio, and sexual selection in Wattled Jacanas (Jacana jacana). However, they did not state anywhere in their paper how they sexed the birds that they studied. They stated (p. 393) that Wattled Jacanas in Panama are sexually monomorphic in plumage and ornamentation. If they sexed them by size, then their conclusions about sexual dimorphism in size would be circular. If they sexed them by behavior, then their conclusions about sex-specific behaviors would be circular. If they sexed them by DNA or some other reliable character, they should have stated this clearly. Although they may well have sexed all the birds reliably, the paper as presented does not exclude the possibility that some large males behaved like females, or that some small females behaved like males. Without a clear statement of what the authors actually did, the reader cannot form an opinion about the validity of the results, and the study cannot be replicated. These comments apply not only to the study by Emlen and Wrege, but more generally to all studies of sexual dimorphism and sexspecific behavior or ecology. Such studies have limited value unless reliable, validated methods are used for sexing, and the criteria used to assign individuals to sex are reported clearly.