The herring gull is a supreme opportunist and scavenger exploiting almost any available food source, including fish offal, refuse, bird chicks, small mammals, eggs, worms and invertebrates (2) (8). Smaller food items are swallowed whole, but larger and harder items are dropped from a height onto rocks to break them open (4). This powerful predator often forages by walking along the shoreline to look for submerged molluscs, but also pirates food from other predatory seabirds, catches fish at sea after plunge-diving into the water, or sits on the water, paddling its legs to propel the body forward, and dips its large beak into the water to catch unsuspecting prey (2) (3) (4). While at sea, the herring gull typically forages in large, widespread flocks that quickly gather around areas of high food abundance, often around feeding whales, dolphins, predatory fish and also fishing boats (3). The herring gull regularly drinks freshwater, but when this is unavailable it will drink saltwater, with excess salt expelled from the body through specialised glands above the eyes (4).
The herring gull may breed in huge colonies of several thousand birds on cliffs, alone, or at the edge of large colonies of other breeding seabirds (8). Typically the male selects the nest site around mid-April, whilst defending it from other birds until a female is found with which to breed (3) (8). A small territory around this nesting site is defended from other breeding birds, usually by the male, and each season established pairs return to the same nest site to breed (3). The nest is a simple depression lined with feathers and vegetation on a cliff ledge or on the ground (8). Usually one to three eggs are laid and incubated for some 28 to 30 days (2) (4). The incubation duties are shared equally between the adult birds, although during times of high food availability, the male does most of the foraging and the female largely stays at the nest (2). The chicks fledge 40 to 45 days after hatching, becoming fully independent 1 to 2 weeks later. Most herring gulls first breed at around 5 years of age and may live for up to 32 years (2).
Outside of the breeding season, the herring gull is a highly gregarious bird and gathers in huge flocks at favoured sites (8). Most adult birds remain close to the breeding site all year round, but the youngest birds tend to migrate southwards. This behaviour is most pronounced in the colder parts of the species’ range, and elsewhere, populations may remain fairly sedentary (3).