Sexing Forster's Terns using morphometric measurements

Forester's Terns (Sterna forsteri), like most seabirds, are monomorphic and are difficult to sex without extensive behavioral observations or genetic sexing. We conducted the first morphological study and discriminant function analysis on Forster's Terns to develop a method to accurately identify their sex in the field. A sample of 84 terns from the San Francisco Bay estuary were captured or collected, measured, and the sex of 40 female and 44 male terns was confirmed by genetic analyses or via necropsy. Male Forster's Terns were larger than females for 7 of 9 morphological measurements, with head-bill length showing the least amount of overlap between the sexes, followed by culmen length and culmen depth at the gonys. Sexual size dimorphism was greatest with retrix R6 length, followed by culmen width, and culmen depth. A discriminant function including only head-bill length accurately sexed 82% of Forster's Terns, whereas a second discriminant function incorporating both head-bill length and culmen depth at the gonys increased sexing accuracy to 87%. When we used a 75% posterior probability or greater of accurately sexing Forster's Terns, we excluded only 18% of the sample that overlapped and accurately sexed 94% of the remaining individuals. Our results indicate that Forster's Terns can be accurately sexed in the field using only 2 morphological measurements.