Nonlocal male mountain white-crowned sparrows have lower paternity and higher parasite loads than males singing local dialect

Bird song often varies geographically, and when this geographic variation has distinct boundaries, the shared song types are referred to as song dialects. We investigated the role of song dialect in male mating success in a wild breeding population of mountain white-crowned sparrows (Zonotrichia leucophrys oriantha). In 2 of 3 years, males singing unusual songs ("nonlocal" males) had lower total fertilization success (measured by microsatellite paternity analysis) than did males singing the local dialect ("local" males). Similarly, females produced disproportionately more young with local than with nonlocal males. However, dialect was not a significant predictor of male mating success when controlling for other factors that might affect paternity. Instead, the low mating success of nonlocal males was apparently due to an interaction between song dialect and parasite load. Nonlocal males were more severely infected by bloodborne Haemoproteus than were local males, although they did not differ in any other measured aspect of quality. Immigrant birds may be immunologically disadvantaged, possibly due to a lack of previous experience with the local parasite fauna, resulting in low mating success. Copyright 2002.