Behavior

Generally highly visible, usually perching at relatively low levels (often <1 m high) on fences, power lines and exposed branches of trees and shrubs (Hilty and Brown 1986, Mobely 2004).

Resident, partially migratory or fully migratory, depending on the population and subspecies; the northernmost populations are permanent residents, with some nomadic and local movements; populations further south in Central America are partially migratory, migrating into South America as far south as northern Brazil; populations breeding in northern and western Colombia may be permanent residents (Mobley 2004). The nominate subspecies migrates from the breeding area in southern South America (Argentina, Uruguay, southern Brazil and southern Bolivia) to central and northern South America to spend the austral winter (March-October; Mobley 2004, Jahn et al. 2013).

Common during migration in Bolivia (northward migration of passage migrants through department of Santa Cruz is February-March and southward migration is September-November; Chesser 1997). Also breeds in low densities in department of Santa Cruz, Bolivia from October-January (AEJ and A. Mamani, unpublished data).

According to Chesser (2005), Fork-tailed Flycatcher is a relatively early spring and fall migrant relative to other Neotropical austral migrants (i.e., species that migrate wholly within South America), arriving on breeding grounds as early as August and on wintering grounds as early as February. After breeding, gather in large flocks in Argentina (Wetmore 1926), then migrate northwards, at times in flocks of thousands. Wetmore (1926) observed one flock of at least 2,000 individuals during fall migration (February) in Uruguay.

Six breeding birds in the province of Buenos Aires, Argentina were deployed with light-level geolocators in 2009 and 2010, and migrated after breeding to northwestern South America (northwestern Brazil, northern Peru and southern Colombia); they then moved eastwards to Venezuela for the rest of the winter (Jahn et al. 2013). Fall migration of these birds lasted approximately 7–12 weeks, at a speed of 45 to 66 km/day, and covered a distance of 2,888–4,105 km (Jahn et al. 2013).

Nesting pairs defend territories from conspecifics (AEJ and DTT, personal observations), though little information is available on territory size and spacing.

The males' aerial courtship display of male consists of a spiral and somersault while calling (Mobley 2004, AEJ personal observations).

Occurs singly, in pairs, family groups or, during migration and the non-breeding season generally, in large flocks of thousands of individuals (Hilty and Brown 1986, Ridgely and Tudor 1994, Mobley 2004, Restall et al. 2006). When flocking, may mix with flocks of Eastern Kingbirds (Tyrannus tyrannus). Actively chases predators, including Spot-winged Falconet (Spiziapteryx circumcincta; Wetmore 1926).

Shiny Cowbirds (Molothrus bonariensis) parasitize Fork-tailed Flycatcher nests (Mobley 2004). At one site in Buenos Aires Province, Argentina, Shiny Cowbirds parasitized 5 of 22 Fork-tailed Flycatcher nests (23%); however, the cowbird eggs were rejected by the Fork-tailed Flycatchers before they could hatch (AEJ and DTT, personal observations).

Motta-Junior (2007) observed a Ferruginous Pygmy-Owl (Glaucidium brasilianum) kill an adult female Fork-tailed Flycatcher in Brazil. American Kestrel (Falco sparverius) also preys on adults (DTT, personal observations). The primary cause of nest failure in central Brazil and eastern Bolivia is nest predation (Marini et al. 2009; AEJ, DTT, A.M. Mamani and V. Bejarano, unpublished data).