Black-headed Gulls are in rapid decline in Norway and is categorized as VU (Vulnerable) on the national Red List. The Norwegian population has decreased from 30 000 pairs around 1990 to just 7000 pairs in 2011. In the study area, the inner Oslo Fjord, the total breeding population counted about 3000 pairs in 2018. This study aims to explain variations in nest and chick survival both within and between colonies, with respect to date of individual nest settlement or colonies establishment, as well as stress elements and colony defence. Multiple colonies were monitored with a combination of nest cameras and drone footage throughout the breeding cycle. From this, data regarding formation, as well as nest and chick survival, were analysed. Of 3050 Black-headed Gull nests monitored in this thesis, one in five produced chicks. Pairs settling early in the breeding season had a much higher nest survival probability than those that settled later. This was true both within and between colonies. Larger colonies had higher nest survival than smaller colonies. The most observable reason for nest and colony failures were predation from Crows, large Gulls, Red Foxes and Badgers. Colonies that failed completely were likely to have a weaker colony defence due to behavioural changes, which made the colony more accessible for predators. Low food availability and a combination of other stress factors are presumed to have made many pairs in the colonies invest less energy in current reproduction and rather focus on own survival and future breeding attempts. From a population perspective, this strategy is sustainable for a few years, but if breeding seasons with low reproduction success are too frequent, the species decline will continue, possible until extinction in Norway.