RESPONSE OF AMERICAN REDSTARTS (SUBORDER PASSERI) AND LEAST FLYCATCHERS (SUBORDER TYRANNI) TO HETEROSPECIFIC PLAYBACK - THE ROLE OF SONG INAGGRESSIVE INTERACTIONS AND INTERFERENCE COMPETITION

Least flycatchers (Empidonax minimus) and American redstarts (Setophag a ruticilla) overlap in the use of food resources on their breeding gr ounds, promoting high levels of interspecific aggression by the social ly dominant flycatcher. We examined the role of song in this interspec ific aggression by using repeated-measures-designed playback experimen ts and observational data on induced aggressive interactions. Flycatch ers were more likely to approach the speaker during presentation of re dstart song than during intervals of no song or presentation of contro l song. Approach was close enough to enable visual contact with a sing ing redstart. In contrast, redstarts made significantly fewer flights following presentation of flycatcher song, when risk of flycatcher att ack may be greatest. Reducing the number of flights likely reduces the risk of flycatcher attack on the redstart, as flycatchers do not atta ck stationary redstart models and are apparently dependent on cues fro m redstart flight for visual heterospecific recognition. Flycatcher-sp ecific responses of redstarts and marked differences in song morpholog y rule out misdirected intraspecific aggression as a proximate or ulti mate cause of interspecific response to song. Results indicate that so ng is an important component in aggressive interactions between these two species, and reflect the dominant role of the flycatcher in such i nteractions. Our results also illustrate the capacity for interspecifi c interference competition to influence behavior and heterospecific so ng recognition in two distant avian taxa.