both an individual chick or a group of chicks begging together. Chicks who are siblings, have learned this behaviour and begin synchronizing their begging signals to decrease the costs as an individual and increase the benefits as a whole. The rate of parental food regurgitation to chicks increases with begging intensity. The amount and response of begging signals differs throughout the nestling period. Usually, there are 3-5 begging events/hour, each lasting around one minute. High intensity begging behaviour appears at the end of the first week in the nest, but the coordination between multiple chicks emerge during the last week of the nestling period. The more siblings present, the more they coordinate their begging while decreasing the number of begging. Sex differences Male chicks have less of a chance of survival when compared to female chicks. 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. Male birds are more likely to be born in the first egg and female birds are more likely to be born in the third. The position of a female black-headed gull in response to the food available when laying the eggs can predict the offspring's characteristics. Conspecific brood parasitism Conspecific brood parasitism is a behaviour that occurs when females lay their eggs in another female’s nest, of the same species. It can reduce the cost of incubation and nestling young by passing it on to another bird. Black-headed gulls usually lay three egg clutches, and the first two are normally larger than the third. The third egg normally has the lowest survival rate, while the first or second are usually the parasitic eggs. Most of the egg dumping occurs within the beginning of the egg laying period. The parasitic eggs being laid in another conspecific’s nest increases the chance of hatching and may occur because of nest desertion or a nest being taken over by another bird. Multiple eggs in a nest from different mothers may also result from intra-specific nest parasitism, joint female nesting, and nest takeover. Intra-specific nest parasitism is a disadvantage to the hosts because the female could end up taking care of the parasitic chicks over her own and therefore neglecting them and reducing their fitness. Another disadvantage for the host is that incubating more chicks than their own takes up more energy. Extra-pair paternity The rate