Risk taking by singing males

The distance at which an individual flees from a potential predator represents a measure of risk taking. If individuals are engaged in another activity that might affect fitness, trade-offs between the fitness benefits of flight and the other activity should determine the nearest distance of approach by a predator. In a comparative analysis of birds, flight distance represented a reliable measure of risk of predation by the sparrowhawk Accipiter nisus that increased with decreasing flight distance across species. To test the hypothesis that singing males adjusted their risk taking to the costs and benefits of early flight, we compared the flight distance of singing and nonsinging birds to an approaching human observing with a binocular. Singing birds on average fled at a greater distance than nonsinging birds, implying that singing birds took small risks. We used a standardized measure of difference in flight distance between singing and nonsinging individuals to investigate factors affecting interspecific variation in risk taking. Species that used more exposed song posts (sites used for singing) took smaller risks than species with less exposed song posts. Species that suffered from higher levels of parasitism as reflected by the prevalence of Plasmodium, but not by 3 other genera of blood parasites, took greater risks during singing compared with nonsinging activities. Likewise, species with high circulating levels of natural antibodies, and hence a history of natural selection caused by bacteria took relatively greater risks during singing than species with few natural antibodies. These findings suggest that risks taken by singing birds have been molded by natural and sexual selection, and that risk taking represents a compromise between the costs and benefits of flight from a potential predator.