SEXUAL ORNAMENTATION, CONDITION, AND IMMUNE DEFENSE IN THE HOUSE SPARROW PASSER-DOMESTICUS

Female mate preferences for extreme male ornaments may arise and be ma intained because males signal their ability to raise an immune defence against parasites by the size of their ornaments. The bursa of Fabric ius is an organ involved in antibody synthesis in young birds, but reg resses before sexual maturity. The relationship between badge size, co ndition, and immune defence was investigated in male house sparrows, P asser domesticus, which have a bib of black feathers under their beak, a secondary sexual character currently subject to a directional femal e mate preference. The size of the bursa of Fabricius decreased during the first year of life in house sparrows. Males with large secondary sexual characters had a smaller bursa than males with small badges. In dividuals with a large directional asymmetry in relative testis size h ad a larger bursa. The abundance of one parasite (a mallophagan) was p ositively related to bursa size. There was a positive relationship bet ween relative volume of bursa of Fabricius and the number of fault bar s on feathers, which is a measure of body condition. These results sug gest that males with, large badges have lower current levels of immune response than males with small badges, and that males in good body co ndition, as evidenced from the absence of fault bars, had a relatively small bursa of Fabricius.