The male sings a three-part dawn song: two short, sweeping whistles (one low- and one high-pitched) and a low burry vibrato. The order of these sylllables can vary, and the whole song lasts about 3 seconds. The two upsweeping whistles resemble the whee-eep call (see Calls), but are more relaxed. The final note is sometimes absent or may be so soft it can only be heard by a nearby mate. Singing can become intense just before dawn: a male within the canopy may sing every two seconds in bouts of 15 to 30 minutes.
Males and females share four basic daytime calls. The most characteristic and frequent call is a loud, penetrating whee-eep whistle that rises quickly and ends abruptly. This call may serve as a contact call between a mated pair or between parents and young. But it is also given as a warning if either parent spots an intruder or predator nearing the nest. The simplest call is a shorter version of the whistle that's usually repeated rapidly 3-10 times. It indicates stress and excitement. A relaxed, slightly burry, vibrato all is given by foraging birds, when flying, or by mates to each other. It's the most common call between mates and may be a locator note. It's also tossed between distant neighbors, but is replaced by other calls if an encounter heats up. These three notes can be modified in pitch and duration, and also be strung together in multi-call bursts or "runs," along with a variety of less stylized calls.The fourth common call, a rasping or stuttering rurr, is the harshest sounding and indicates stress and alarm.
When alarmed or very excited, including while repelling predators or pursuing intruders, Great Crested Flycatchers snap their bills loudly.