Sex determination by morphology in New Holland Honeyeaters, Phylidonyris novaehollandiae: contrasting two popular techniques across regions

Sex determination of individuals is often required for ecological and behavioural studies but is difficult to carry out in the field for species that are only slightly dimorphic. To address this issue, researchers may use a variety of methods that rely solely on morphological measurements for sex determination. There are two main groups of morphological methods; (1) based on discriminant analysis, and (2) based on resolving mixed-modal distributions. Here, we use one method from each of the two groups to sex the slightly dimorphic New Holland Honeyeater, Phylidonyris novaehollandiae, in South Australia, and we compare results of the two methods in relation to a genetic standard. We found that performance of both methods was comparable, but varied between populations. We also found regional differences in the best discriminating variables for morphological sex determination. This regional variation in performance of methods indicates that a single method for morphological sex determination cannot be applied across regions, even within species; furthermore, average morphological trait values should be reviewed across years given the possible role of selection or drift to influence phenotype. We suggest that when accurate sex discrimination is important, an alternate, reliable method, such as anatomical or genetic identification, should be favoured above morphological methods.