This individual is showing a full breeding plumage black cap. Non-breeders have much reduced black on the crown. This species was formerly included in the genus Sterna but is now placed on its own in the genus Gelochelidon. The race G. n. macrotarsa is the largest and palest of the six currently recognized subspecies. As its name implies it also has much longer legs than other races. The new HBW-Alive splits Australian breeding birds into a separate species called Australian Gull-billed Tern (Gelochelidon macrotarsa). This split is based on a scoring system advocated by Tobias et al. (2010). This taxon gets "scoring points" for having a different bill shape, more decurved culmen and no gonydeal angle as seen in this photo. The Tobias system has been adopted by Birdlife International which also splits this species. However neither Clements nor IOC have followed suit; both leave macrotarsa as a subspecies.
Digiscoped with Panasonic DMC-LZ5 | Nikon FieldScope III | 30XWA | hand-held (no adapter)