Flight songs of Dusky Flycatchers: a response to bird-hunting raptors?

Abstract

Flight-song displays usually include distinctive vocalizations and behaviors that are probably energetically expensive and also likely to attract the attention of aerial predators. We observed flight songs performed by Dusky Flycatchers (Empidonax oberholseri) in two breeding populations. During 15 yr at Tioga Pass, California, we observed the display twice and, during 9 yr at Steamboat Mountain, British Columbia, we observed 21 flight-song displays. These displays were generally observed later in the breeding season, suggesting that they were not used for courtship, mate attraction, or territory defense. However, for 83% of these displays, a raptor was observed to be present, usually either an American Kestrel (Falco sparverius) or a Sharp-shinned Hawk (Accipiter striatus). We hypothesize that these displays, like stotting or other pursuit-deterrent signals, may be directed at small raptors that represent a greater threat to Dusky Flycatchers and might deter attack by signaling awareness. However, because only males were observed performing flight-song displays, it is also possible that these displays serve to alert mates and offspring about the presence of an aerial predator and reduce their vulnerability to attack. RESUMEN La conducta de cantar mientras se vuela incluye vocalizaciones distintivas y conductas que probablemente son energeticamente costosas y que pudieran atraer la atencion de depredadores. Observamos cantos al volar en dos poblaciones del papamoscas Empidonax oberholseri. Durante 15 anos de estudios en Tioga Pass, California y nueve anos en Steamboat Mountain, Columbia Britanica, observamos la conducta previamente descrita observamos la conducta en dos y 21 ocasiones, respectivamente. Los cantos a vuelo fueron observados tardes en la epoca reproductiva, lo que sugiere que no fueron utilizados para el cortejo, atraer la atencion de parejas prospectivas o defender el territorio. Sin embargo, en el 83% de las ocasiones, se observaron tambien a individuos de Falco sparverius o de Accipiter striatus. Proponemos como hipotesis, que la conducta observada, al igual que otras conductas de proteccion, pueden ser dirigidas a rapaces pequenos que representan una amenaza a los papamoscas y que pudieran detener el ataque indicando precaucion. Sin embargo, dado el caso de que solo se observaron machos del papamoscas exhibiendo la conducta, es probable ademas que la conducta sirva para alertar a las parejas y pichones de los vocalizadores sobre la presencia de un depredador y de esta manera reducir su vulnerabilidad de ataque.