The silver gull is a gregarious bird (3) (4), and is often found in large flocks of hundreds to thousands of individuals (3). This species generally roosts at inland sites around the margins of wetlands or floodwaters (3), but it is known to wander widely outside the breeding season (2) (6). Seasonal movements have been recorded, with some populations of the silver gull moving short distances from the colony to nearby coastlines. Southern and eastern populations generally migrate northwards, while western populations tend to move southwards (2).
An opportunistic feeder (6), the silver gull has an extremely varied diet, including fish, marine and terrestrial invertebrates, seeds, berries, and even bird eggs (2) (3) (6), including those of the great crested tern (Sterna bergii) (7). The silver gull's foraging habits are as diverse as its diet, and it has been observed 30 kilometres offshore feeding on amphipods at the water’s surface (2), as well as scavenging through human refuse on land (3) (4), or feeding on the wing as it hawks after swarming insects (2) (3). The silver gull is known to be a kleptoparasite (2) (6) (8), stealing food from other species, particularly terns and pelicans (2) (8).
The silver gull breeds on small islands (4) (6), mainly offshore but also on freshwater or brackish lakes (3) (6), and it is known to nest close to tern colonies (2). Although it can occasionally nest in solitary pairs, the silver gull is generally a colonial species (2) (3) (6). Colonies in tropical areas tend to be small, usually comprising between 3 and 25 pairs. In the southern parts of its range, silver gull colonies can contain up to 3,000 breeding pairs, although the colony size is limited by the availability of food (2) (6).
The timing of breeding in the silver gull varies depending on the location and also on the age of the bird (6), with older birds usually nesting earlier and producing more young. In Western Australia, egg laying occurs from March to November, and some pairs may go on to raise a second brood in the same season (2).
The nest of the silver gull is a shallow cup (2), made from grass and other suitable material. It is built in tree stumps or on embankments of wetlands (3), although in the Capricorn Group in the Great Barrier Reef this species tends to nest on sandy or rocky ground with low vegetation. The female silver gull lays a clutch of between one and five eggs, with three being most common (2). The eggs are laid at intervals of two to four days and are incubated by both sexes (3). The incubation period is between 21 and 27 days, and the chicks remain within the colony for a further 4 weeks (2), until the adult birds lead them away (2) (3). The young silver gulls gather in small groups, and generally become capable of flight between five and seven weeks of age (3).
Silver gulls are usually able to breed at four years of age (2), although breeding may sometimes occur as early as two years old (3). The silver gull is thought to be able to continue breeding for about 11 years (2).