of extra-pair paternity (EPP) has a large variation between populations of black-headed gulls. It is primarily a context-dependent strategy, meaning not all black headed gulls experience this behaviour. The variation between populations of extra-pair paternity can be explained by the variation it has on the advantages and disadvantages it has on a female, as well, as the variation in pressure on a females choice. The differences in the rate of EPP may be determined by multiple different factors: life history traits, ecological factors or different behavioural strategies of males. Central–periphery gradient within colonies Egg-laying can be earlier in Black-headed Gulls nesting in the centre of the colony, with central pairs tending to lay larger eggs, which have a higher hatching success, than pairs nesting at the periphery of the colony. Centrally nesting individuals have also been found to be in better condition and have higher genetic quality. Walking displays Black-headed gulls display both head-bobbing walking (HBW) and non-bobbing walking (NBW). Head-bobbing walking is expressed by a hold phase and a thrust phase. The hold phase in black-headed gulls occurs mainly during the single support phase and is when the bird balances its head to equal the environment. Head-bobbing walking occurs during a seeking type foraging by walking through water and includes benefits such as enhancing motion and pattern detection and gathering depth information from motion parallax during the thrust phase. Non-bobbing walking occurs when black-headed gulls are displaying a waiting behaviour while foraging on flat surfaces. Uses The eggs of the black-headed gull are considered a delicacy by some in the UK and are eaten hard boiled. The collection of black-headed gull eggs is heavily regulated by the UK government. Eggs may only be taken by a small number of licensed individuals at six sites between April 1 and May 15 each year and only a single egg may be taken from each nest. No eggs are permitted to be sold after June 30. 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. Synchronization Observations on the behavior of black-headed gulls show that black-headed gulls individuals synchronize their vigilance activity with other black-headed gulls neighbors. Synchronization in black-headed gulls groups is dependent on the distance between the black-headed gulls members. Australian discovery On 19th October 1991, local Broome birder Brian Kane saw a