1. Culls are being performed in many yellow-legged gull Larus cachinnans colonies in the Mediterranean region, but little is known of their effects on this species. Between 1992 and 1996, 25 000 breeding yellow-legged gulls were culled in the colony of the Medes Islands, north-western Mediterranean, because of the possible role of gulls in the transmission of pathogens and as predators on other bird species. In the present study the effect of the culls on several ecological parameters and on the population dynamics of the colony were analysed.
2. Some breeding parameters (nest density, clutch and egg sizes, chick growth and breeding success, intraspecific predation, size and body condition of adults), diet (importance of the different prey categories and width of the trophic niche) and population dynamics of the colony were analysed during the years of cull. The annual culls did not include the entire colony, so culled and unculled areas were distinguished.
3. Nests density decreased significantly both in the culled and the unculled areas. No significant difference in clutch size was detected between culled and unculled areas in any year, but clutch size decreased significantly through time within the culled areas. In three-egg clutches the mean egg volume was significantly larger in unculled than in culled plots, whereas no significant year effect was found. No differences were detected in the mass or in the body condition of chicks throughout the study, whereas both fledging success and breeding success increased significantly over the period of the study. Intraspecific predation had decreased significantly by 40% two years after the beginning of the study. Size and body condition of adults varied between years but no trend was observed.
4. Despite the large decrease in breeding gull numbers and an expected reduction in intraspecific food competition, no changes in diet were detected during the study. The dietary niche width was very similar between years, and gulls continued to exploit the same foraging resources.
5. Annual censuses showed that from 1960 to 1992, colony size increased at a rate of 5% per year. During the culling period, between 21% and 29% of the breeding adults were killed, and colony size decreased progressively at an estimated rate of 19% per year. The estimated annual adult survival according to ring recoveries during the culling period was 74%. A demographic model was developed to assess the observed changes in numbers of yellow-legged gulls. This suggested that gulls born in the Medes Islands were emigrating during the years of culling, with a particularly high estimated emigration rate of 25% per year since 1994.
6. The planning of culling programmes by wildlife agencies has not always taken into account the multiple factors responsible for the population dynamics of colonies and the effects of culls. Culling at the Medes Islands probably failed to reduce the breeding numbers of yellow-legged gulls at the metapopulation level, due to the emigration of birds to neighbouring colonies that had recently increased in numbers. Thus, the potential problems linked to large numbers of gulls in a colony may simply have been transferred to other sites.