[4] The genus name Chroicocephalus is from the Ancient Greek words khroizo, "to colour", and kephale, "head".
[5] In flight, the white leading edge of the outer wing (the outer primaries) is a good field mark, particularly combined with the dark underside of the inner primaries, which distinguishes it from the white underside of those feathers in its close relative the Bonaparte's gull.
Juvenile birds, for the first month or two after fledging, have a mottled pattern of brown spots over most of the body, and a black band on the tail; in late summer, they moult into first-winter plumage, with a grey back, but retaining a brown carpal bar on the inner wing, blackish secondary feathers, and the black band on the tail.
[5] Black-headed gulls breed across the Palearctic over much of Europe and northern Asia, from Iceland and Ireland east to Japan and eastern China.
Areas within the breeding range with milder winters such as Great Britain receive large influxes of migrants from colder areas like Scandinavia, Poland, the Baltic States, and Russia; the British winter population is around 3 million.
[5][10] Small numbers occur in winter in northeast North America as far south as Virginia, often in flocks of the similar-looking Bonaparte's gull.
It eats insects, fish, seeds, worms, scraps, and carrion in towns, or invertebrates in ploughed fields with equal facility.
Some of these include removing eggshells from the nest after hatching, begging co-ordination between siblings, differences between sexes, conspecific brood parasitism, and extra-pair paternity.
[16] Eggshell removal is a behaviour seen in birds once the chicks have hatched, observed mostly to reduce risk of predation.
Chicks who are siblings, have learned this behaviour and begin synchronising their begging signals to decrease the costs as an individual and increase the benefits as a whole.
Black-headed gulls are a sexually size-dimorphic species, so the larger sex is at a disadvantage when the amount of food sources are low.
The position of a female black-headed gull in response to the food available when laying the eggs can predict the offspring's characteristics.
[32] Non-bobbing walking occurs when black-headed gulls are displaying a waiting behaviour while foraging on flat surfaces.
[33] The eggs of the black-headed gull were considered a delicacy by some in the UK and eaten hard boiled.
As the gulls tend to lay in late April and early May, the eggs are only available to purchase for 3 or 4 weeks per year.
Injured by a farm cat and left behind during the seasonal migration, Kehaar finds himself stranded on the Downs and is taken in by a warren of rabbits.
[39][40][41] True to Adams' stated intentions of trying to keep their behaviour close to reality, Kehaar is characterised as intelligent, gregarious, noisy, messy, and impatient, and with a guttural accent.
[39] Kehaar appears in all three screen adaptations of the novel; the character was voiced by Zero Mostel in the 1978 film, Rik Mayall in the 1999 TV series, and Peter Capaldi in the 2018 miniseries.