PARASITE VIRULENCE AND HOST IMMUNE DEFENSE - HOST IMMUNE-RESPONSE IS RELATED TO NEST REUSE IN BIRDS

The evolution of parasite virulence has been hypothesized to be relate d to the mode of parasite transmission; horizontally transmitted paras ites can afford to damage their hosts more than vertically transmitted parasites because increased virulence does not reduce the probability of transmission to new hosts. This relationship between mode of trans mission and virulence would particularly select for improved immune de fense in hosts that are subject to horizontally transmitted parasites. Among avian hosts, hole nesters and colonial nesters frequently reuse nest sites because of nest-site limitation, and this results in an in creased frequency of horizontal transmission. Comparison of the size o f two organs involved in the immune defense between pairs of bird spec ies being either hole or open nesters, or colonially or solitarily nes ting birds, respectively, revealed that the size of the bursa of Fabri cius and the spleen were consistently larger in hole nesters than in o pen nesters, and similarly in colonially breeding bird species than in solitarily breeding species. These results support the hypothesis tha t mode of parasite transmission affects the evolution of immune defenc e in hosts.