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The pale birds that circle round the Sydney Harbour Bridge at night are Silver Gulls, Larus novaehollandiae.
Although they can be seen all year round, they will appear in greater numbers when moths are abundant. This is because the moths are attracted to the bridge lights and the gulls take advantage of this food supply. The gulls may also be attracted to the lights themselves.
Flying at night is not unusual for Silver Gulls – they are known to feed at dusk, catching insects around water sources and lights. They also follow the harbour ferries (during the day and at night), possibly to take advantage of the slipstream but perhaps also to catch small marine animals churned up in the ferries’ wakes.
Silver Gulls are highly adaptable and are one of the few birds to have benefited from people.
It is also possible to see bats flying over the Bridge at night – the large bats are Grey-headed Flying Foxes (or fruit bats), Pteropus poliocephalus, which are flying to or from their camps nearby to fruiting or flowering trees in other parts of Sydney. Flying Foxes can be distinguished from the gulls as the bats are unlikely to be circling around the Bridge in the same way as the gulls do, as there is no food for them there.
Much smaller insectivorous bats – or Microchiroptera – also fly around the Bridge catching insects; they have a rapid, erratic flight and are much smaller than the fruit bats, so will be much harder to see.