Risk taking by singing males

The distance at which an individual flees from a potential predatorrepresents a measure of risk taking. If individuals are engagedin another activity that might affect fitness, trade-offs betweenthe fitness benefits of flight and the other activity shoulddetermine the nearest distance of approach by a predator. Ina comparative analysis of birds, flight distance representeda reliable measure of risk of predation by the sparrowhawk Accipiternisus that increased with decreasing flight distance acrossspecies. To test the hypothesis that singing males adjustedtheir risk taking to the costs and benefits of early flight,we compared the flight distance of singing and nonsinging birdsto an approaching human observing with a binocular. Singingbirds on average fled at a greater distance than nonsingingbirds, implying that singing birds took small risks. We useda standardized measure of difference in flight distance betweensinging and nonsinging individuals to investigate factors affectinginterspecific variation in risk taking. Species that used moreexposed song posts (sites used for singing) took smaller risksthan species with less exposed song posts. Species that sufferedfrom higher levels of parasitism as reflected by the prevalenceof 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 bacteriatook relatively greater risks during singing than species withfew natural antibodies. These findings suggest that risks takenby singing birds have been molded by natural and sexual selection,and that risk taking represents a compromise between the costsand benefits of flight from a potential predator.