CONICET | Buscador de Institutos y Recursos Humanos

ABSTRACT.?The Alder (Empidonax alnorum) and Willow (E. trailli) flycatchers are cryptic species, and theirdistribution outside the breeding season is poorly known, owing mostly to identification difficulties. Our new records suggestthat large numbers of Alder Flycatchers overwinter in rivers crossing the Chaco region and in the foothill forests of theYungas of Argentina, significantly increasing their southern wintering range. Records in northern Argentina span 3November to 23 March. Key habitat for overwintering Alder Flycatchers in Argentina included stands of palo bobo (Tessariaintegrifolia) along the R´ıo Bermejo and tributaries, either in association with sparse shrubs of chilca (Baccharis salicifolia) ormore rarely with canebrake (Gynerium sagittatum). Other habitats used were old shrubby ?madrejones? with Tessaria scruband sacha caf´e (Sesbania virgata), very dense Baccharis salicifolia scrub next to flowing creeks in foothill Yungas and dryChaco, and riparian forests dominated by palo flojo (Albizia inundata) and timb´o (Enterolobium contortisiliquum). Seasonalflooding of these habitats did not affect the presence of Alder Flycatchers. Small territories of c. 20320 or 25325 m weredefended in Tessaria stands. Alder Flycatchers fed mostly on insects in flight (aerial hawking), but also on green Lepidopteralarvae (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) weresimilar to those used while breeding. Alder Flycatchers collected in Argentina had significantly longer wings and wider billsthan specimens from the sympatric Euler?s Flycatchers (Lathrotriccus euleri) from Argentina with which it has beenconfused in museum specimens. Received 11 November 2015. Accepted 31 January 2016.