The Alder (Empidonax alnorum) and Willow (E. trailli) flycatchers are cryptic species, and their distribution outside the breeding season is poorly known, owing mostly to identification difficulties. Our new records suggest that large numbers of Alder Flycatchers overwinter in rivers crossing the Chaco region and in the foothill forests of the Yungas of Argentina, significantly increasing their southern wintering range. Records in northern Argentina span 3 November to 23 March. Key habitat for overwintering Alder Flycatchers in Argentina included stands of palo bobo (Tessaria integrifolia) along the Río Bermejo and tributaries, either in association with sparse shrubs of chilca (Baccharis salicifolia) or more rarely with canebrake (Gynerium sagittatum). Other habitats used were old shrubby ‘madrejones' with Tessaria scrub and sacha café (Sesbania virgata), very dense Baccharis salicifolia scrub next to flowing creeks in foothill Yungas and dry Chaco, and riparian forests dominated by palo flojo (Albizia inundata) and timbó (Enterolobium contortisiliquum). Seasonal flooding of these habitats did not affect the presence of Alder Flycatchers. Small territories of c. 20 × 20 or 25 × 25 m were defended in Tessaria stands. Alder Flycatchers fed mostly on insects in flight (aerial hawking), but also on green Lepidoptera larvae (upward sally-strikes), and on insects on leaves, and ripe fruits of tala (Celtis cf. ehrenbergiana) in forest understory (clinging). Vocalizations given by overwintering birds (fee-bee-oo, zwee-oo, wee-oo, churr, pit, double-peak, and kitter) were similar to those used while breeding. Alder Flycatchers collected in Argentina had significantly longer wings and wider bills than specimens from the sympatric Euler's Flycatchers (Lathrotriccus euleri) from Argentina with which it has been confused in museum specimens.
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