In many passerines, rates of extra-pair paternity are high, but relatively few studies have examined the behaviors females use to obtain extra-pair copulations. We studied extra-pair behavior in females of the Acadian Flycatcher (Empidonax virescens), a species in which 58% of females produce extra-pair young and males often sire offspring on distant territories. We used radiotelemetry to document the extent and frequency of females' forays away from their territories and used behavioral observations and playback experiments to test if females' vocalizations advertise their fertility. No radio-tagged females left their territories (n = 12 females, 105 hr of tracking) even during the hour before dawn. Paternity analyses revealed that at least six of these females produced extrapair young. Focal observations revealed that fertile females spent significantly more time calling “chiff,” gave more calls per hour, and called in longer bouts than did incubating females. Extent of females' chiffing during their fertile period was not related to subsequent extra-pair paternity in their nest (n = 12). We tested for a territory-defense function of chiffing by playing chiff calls to females at both the fertile and incubation stages. Females responded to playbacks on their territory by approaching or flying over the speaker, did not increase their chiffing rate significantly, but did give other high-intensity calls. Our results suggest that extra-pair fertilizations of Acadian Flycatchers occur primarily on a female's own territory and that extra-pair males could use females' vocalizations to locate fertile females.