Greater Crested Tern
Thalasseus bergii

A fairly common tern, generally found in the southern hemisphere and Indian Ocean. The Thalasseus terns were recently split from the other Sterna terns and include species such as the Caspian and Sandwich Terns and often have shaggy crests and large beaks. The first two birds are in their winter plumage - in summer, they have a shaggy black crest, as seen in photos 3-5. In photo 5, the two birds are courting.
 

1: Coogee, NSW, 27/06/2010.
2: South Pacific, off Sydney, 13/03/2010.
3: South Pacific, off Sydney, 14/08/2010.
4 and 5: Newcastle, NSW, 15/08/2010.