Sea gulls can be very aggressive birds even to the point of stealing food from a childs hand. The most common is the herring gull with the distinctive pink legs and yellow bill, but the are also the lesser black backed and greater black backed gulls which can also be a nuisnace. Gulls mate for life and will retun to a breeding spot year after year and their offspring will also join them once they reach sexual maturity at the age of 4. This could be behind your chimney or a flat roof and after a few years this can cause expensive damage such as water ingress, damage to roofs etc. early autumn or in the early part of the year when they are not nesting, proofing can be carried out to prevent them from taking up residence.