Volume 49, No. 1

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Morphometric sex determination of breeding adult Royal Thalasseus maximus and Sandwich terns T. sandvicensis in Louisiana


1Nicholls State University, 906 E. 1st Street, Thibodaux, Louisiana 70310, USA
2Louisiana State University and AgCenter, Baton Rouge, Louisiana 70803, USA
3Current address: Audubon Florida, Florida Coastal Island Sanctuaries, 410 Ware Blvd #702, Tampa, Florida 33619, USA
4Current address: Southern Illinois University, 1263 Lincoln Drive, Carbondale, Illinois 62901, USA
5Current address: Louisiana Oil Spill Coordinator's Office, Baton Rouge, Louisiana 70806, USA
6Current address: Ducks Unlimited, Inc., 125 Southpark Rd., Lafayette, Louisiana 70508, USA (


NEPSHINSKY, M., TAYLOR, S.S., LIECHTY, J.S., MINOR, A.K., WINDHOFFER, E.D. & PIERCE, A.R. 2021. Morphometric sex determination of breeding adult Royal Thalasseus maximus and Sandwich terns T. sandvicensis in Louisiana. Marine Ornithology 49: 127 - 132

Received 23 September 2020, accepted 07 January 2021

Date Published: 2021/04/15
Date Online: 2021/04/10
Key words: molecular sexing, monomorphic, Polymerase chain reaction (PCR), seabird, sexual size dimorphism


Many seabirds have sex-specific traits, including survival, philopatry, behavior, and diet, information that is essential for effective conservation strategies. The difficulty of sexing monomorphic seabirds, however, has resulted in a lack of information on these important traits and an incomplete understanding of the ecology of many species. We used a polymerase chain reaction (PCR)-based sexing approach to determine if Royal Thalasseus maximus and Sandwich terns T. sandvicensis in Louisiana can be accurately sexed using morphometrics. DNA samples and morphological measurements were obtained from Royal Terns (n = 106) and Sandwich Terns (n = 112) to accurately identify sex and compare gender differences in morphological measurements. In both species, males had greater bill length and head + bill length relative to females. Sexual size dimorphism ranged from 0.0%-4.7% in Royal Terns and 0.4%-3.5% in Sandwich Terns. Using discriminant function analysis, equations correctly assigned sex in 75% of Royal Terns using head  + bill and wing chord measurements and in 82% of Sandwich Terns using head + bill and mass measurements. Our methods provide an accurate and economical field-sexing technique for Royal and Sandwich terns, enabling research into sex-based differences in behavior, physiology, and ecology.


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