Density dependence in seabirds: Great SkuasStercorarius skuastart to breed at a younger age when conditions are better

Large-scale and long-term ringing and cohort colour ringing of chicks, combined with trapping of immatures on clubs and adults on nests, allows the age of first breeding by Great Skuas Stercorarius skua to be investigated. Modal and median age of first breeding of Great Skuas at their largest colony (Foula, Shetland) was seven years, but there was considerable individual variation. Birds may first breed when four to >11 years old. Compared with the situation in Foula, Great Skuas start breeding at a younger age at smaller colonies. When conditions at Foula deteriorated, resulting in reduced adult survival, reduced breeding success and population decline, age of first breeding increased. These patterns imply a density-dependent age of first breeding, but with individual variation because higher-quality birds recruit at a younger age than birds in poor body condition. Density-dependent variation in age at first breeding may have an important influence on population growth rate, and this would merit study also in other species of seabirds through a dedicated ringing and colour-ringing programme.