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Measurements
Substantial overlap among 3 subspecies (antillarum, athalassos, browni) and between sexes within subspecies for many morphological measurements (Thompson et al. 1992; see Appendix). S. a. antillarum tends to have longest exposed culmen and browni longest wing-chord, but these are not diagnostic. Collectively, exposed culmen, lower mandible, and back color (reflectance value) >90% reliable in distinguishing sex of each subspecies (Thompson et al. 1992); unfortunately, sample sizes of available specimens for California and interior populations were relatively small (<30 for each sex and locality).
Linear
Wing, tail fork, bill, bill depth at gonys, bill depth at rear edge of nostrils, and tarsus measurements for 180 museum specimens from s. U.S. and West Indies showed no constant differences (Olsen and Larsson 1995).
Bill Length
24–32 mm (average about 28); sexes similar.
Wing Length
153–180 mm (average about 167); sexes similar.
Tail Length
60–80 mm, but variable with age of bird.
Tarsus Length
14–17 mm (average about 15.5).
Mass
Limited data suggest no gender differences (see Appendix), but females more variable, probably because of egg production. Few data for comparisons across species’ range. No data on seasonal shifts. Breeding Least Terns in Netherlands Antilles (sample size not specified) range from 30 to 45 g (Olsen and Larsson 1995). In Texas, 37 live adults averaged 43.8 g (range 39.0–51.5; BCT). In Massachusetts, 75 males averaged 46.9 g (range 41–54; JLA). In Nebraska, 38 adults averaged 44.3 g (range 36–53.5 g; EMK).
Thompson, Bruce C., Jerome A. Jackson, Joannna Burger, Laura A. Hill, Eileen M. Kirsch and Jonathan L. Atwood. 1997. Least Tern (Sternula antillarum), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/290
