A medium sized tyrant flycatcher with dark upperparts and white underparts. Black on head, wings and tail, with gray back, though the color of the back varies from darker to lighter gray, depending on the subspecies (see Geographic Variation). Distinctively long, bifurcated tail that is often longer than the body (longer in males than in females).
Adults in fresh plumage are almost unmistakable due to the long, bifurcated tail, though molting adults and juveniles can be confused with Eastern Kingbirds (Tyrannus tyrannus) in worn plumage. Eastern Kingbirds in fresh plumage, however, have a white terminal band on the tail, which Fork-tailed Flycatcher lacks.
Adult: Most of head and upper nape black with semiconcealed yellow crown patch; upperparts gray, uppertail coverts black and wings blackish brown; wing coverts and secondaries edged pale gray to whitish; tail black with extremely elongated outer rectrices that are basally edged white; underparts white (Mobley 2004). Sexes similar except for length of tail, which is longer in males.
Juveniles duller than adults with much shorter tail, especially compared to adult males, with sooty gray head, more brownish back, and pale cinnamon margins on upper tail and wing coverts (Mobley 2004). Juveniles and immatures may also have some brown on crown and lack or have reduced yellow crown patch (Hilty and Brown 1986, AEJ and G.I. Giraldo, personal observation).
The nominate subspecies primarily molts Apr-July, subspecies monachus molts primarily July-September, and the molt of subspecies sanctaemartae presumably is similar to the nominate subspecies (Pyle 1997). Most captured individuals of the nominate subspecies in central Colombia (Department of Meta) in June and July were molting tail and primary feathers (AEJ and J.I. Giraldo, unpublished data). In Buenos Aires, Argentina, adults of the nominate subspecies begin molting at the end of the breeding season (i.e., late January; DTT, personal observations).
Tarsi and toes: blackish
Bare parts color data from Mobley (2004). Also, juveniles have a yellowish gape (AEJ and DTT, personal observations).
Total length of adult male is 37-40.5 cm, and of adult females is 28-30 cm (Mobley 2004).
At a site in Buenos Aires Province, Argentina, mean measurements of adult males (subspecies savana) are: wing chord (unflatenned): 110 mm (±3 SD; n = 42), tail: 225 mm (±28; n = 40); mass: 32 g (±2; n = 44). Mean measurements of females (subspecies savana) at the same site are: wing chord (unflatenned): 103 mm (±3; n = 35), tail: 172 mm (±23; n = 32); mass: 30 g (±3; n = 37).
Eggs measure ca 22 x 16 mm (de la Peña 1987, Mezquida 2002, Marini et al. 2009, DTT unpublished data). Eggs have a volume of 3.01±0.07 cm3 (n = 17 nests) at a site in Buenos Aires Province, Argentina (DTT, unpublished data).