The water rail, Rallus aquaticus, has no visible sexual dimorphism. In previous studies, where morphometric measurements and molecular data had been used, a sex-discriminating formula was described from a German water rail population. We tested this formula in 21 water rails from a Romanian population and found that 90% of them were correctly sexed. A new formula was calculated specifically for the new population, and this one had a 95 % correct discriminating score in our population and 67.7% in the German population. The data analysis had revealed that these two populations were significantly different in terms of body measurements and the reasons for this difference were discussed. We concluded that the formula calculated in the German population had a higher resolution, being established from a larger number of individuals with a lower degree of variance, and therefore it may have a high validity in sex discrimination in other populations. The formula developed for the Romanian population has a high discriminant level only in a local scale and a low resolution when used on a population with high variability.

Based on plumage traits it is impossible to reliably identify the sex of Water Rail Rallus aquaticus and Spotted Crake Porzana normal. In order to analyze their sexual size dimorphism we collected morphometric data and, for genetic control, feather pins as DNA source of 71 adult Water Rails and 31 Spotted Crakes during the breeding season 2008-2009. We determined the sex of each individual using PCR based molecular techniques taking into account the length polymorphism of the sex linked CHD1Z allele encountered in Water Rails. The polymorphism of this sex marker may have caused erroneous conclusions in previous studies using different approaches. In the present case, unawareness of the polymorphism would have lead to the misclassification of 50.8% of the individuals, which would result in problems with morphological comparison of the sexes. In general, males of both species were significantly larger than females. However, single measurements partly showed a high overlap of the sexes. Conducting a stepwise discriminant function analysis revealed bill, maximum wing chord and tarsus length as best discriminants for Water Rail. The resultant discriminant function, which allows assigning an individual to one of the sexes with a specific accuracy by its body measurements, correctly classified 98.6%. In Spotted Crakes the analysis included tarsus and wing length into the model and assigned the sex with an accuracy of 100%. The discriminant functions thus represent a simple and cost efficient way to determine the sex of these rallids for field ornithologists.