An Evaluation of Morphological Methods for Sex Determination of Black Skimmers (Rynchops niger) in the Pre-fledgling Stage

Abstract

Black Skimmers (Rynchops niger) are sexually monochromatic seabirds that are dimorphic in size. Although most dimorphic seabird adults can be accurately sexed using non-invasive techniques, determining the sex of pre-fledglings is more challenging. Schew and Collins (1990) developed an equation to estimate chick age from wing chord and used body mass of chicks = 24 days old to evaluate the sex of skimmers in California; however, this was not verified on known-sexed individuals. Our objective was to assess the accuracy of their methodology on banded skimmer chicks from southwest Florida whose sexes were confirmed during adulthood. Out of 32 skimmers, Schew and Collins’ (1990) method only classified 44% of males and 81% of females correctly. While males on average were larger than females, there was considerable overlap between the sexes when measurements of body mass, culmen and tarsus length were plotted against wing chord. It appears our Florida chicks grow slower than those in the California study and do not asymptote until after they are flight capable. As Black Skimmer populations decline, further research is needed to determine the most effective chick sexing method as knowledge of survival rates of pre-fledglings by sex will be critical for evaluating management efforts. Received 12 Feb 2023, accepted 7 Aug 2023.