Populations of Common terns were greatly reduced in the 19th century because of habitat loss due to development, hunting and egging by people, and also because of the millinery trade, where feathers, wings, and sometimes whole stuffed birds were used as hat decorations. Conservation initiatives greatly eased such pressures and the species recovered in the first half of the 20th century. However, starting in the mid-20th century, populations have started to decline again, likely due to a combination of factors such as competition for nesting sites by gulls (whose populations blossomed near garbage dumps), disturbance by people and off-road vehicles, and storm floods. The Common Tern is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds and the U.S.-Canada Migratory Bird Treaty apply. After the eggs are laid, some lining material is added throughout the incubation period, such as grass, reeds, or even rubbish. Clutch size is normally three eggs, fewer in bad food years. They are cream, buff, or brown, and finely marked with streaks or spots of black, brown or grey. Incubation is by both sexes and lasts 22–33 days, depending on disturbances at the colony which may leave the eggs unattended.