The evolution of song repertoires and immune defence in birds

Song repertoires (the number of different song types sung by a male) in bir ds provide males with an advantage in sexual selection because females pref er males with large repertoires: and females may benefit because offspring sired by preferred males have high viability. Furthermore, males with large repertoires suffer less from malarial parasites, indicating that a large r epertoire may reflect health status. We hypothesize that sexual selection m ay cause a coevolutionary increase in parasite virulence and host immune de fence because sexual selection increases the risk of multiple infections th at select for high virulence. Alternatively a female mate preference for he althy males will affect the coevolutionary dynamics of host-parasite intera ctions by selecting for increased virulence and hence high investment by ho sts in immune function. In a comparative study of birds, repertoire size an d relative size of the spleen, which is an important immune defence organ, were strongly, positively correlated accounting for almost half of the vari ance. This finding suggests that host-parasite interactions have played an important role in the evolution of song repertoires in birds.