Gull-billed Terns (Sterna nilotica) can often be found at the Mullens roost with the migratory shorebirds. These Terns stay in Australia all year but leave this area to breed on inland lakes and water. In breeding plumage they have a full black cap. In non-breeding plumage this black cap reduces to a black patch around the eye. There were several birds still with a black cap in the flock I saw the other day. They had been roosting with Bar-tailed Godwits.
Before I got very close most of them flew off a little way to another part of the roost.