House sparrows were individually marked with unique identifying rings, weighed, aged and sexed based on plumage and skull pneumatization characteristics . House sparrows were classed as: 1) yearlings, birds in their first year of life; 2) adults, birds in at least their second year of life and 3) birds that could not be classified, including adult birds and yearlings with a completed moult and ossified skull pneumatisation . Blood samples were obtained from the jugular vein of each bird using sterile syringes. Samples from yearlings were used for the molecular identification of the sex of individuals. The volume of blood extracted depended on bird size but never exceeded 1% of avian body mass. Blood was collected in Eppendorf tubes and maintained in cold boxes in the field and at 4°C for 24 hours. After that, blood was centrifuged for 10 minutes at 4,000 rpm to separate the serum and cellular fractions. Samples were frozen at -20°C until subsequent analysis.