MIGRATION OF THE WILLOW FLYCATCHER ALONG THE MIDDLE RIO-GRANDE

We studied timing, abundance, subspecies composition, fat stores, stop over length, and habitat use of Willow Flycatchers (Empidonax traillii ) during spring and fall stopover along the Middle Rio Grande, New Mex ico. Spring migration started in mid-May and lasted about a month. Fal l migration started in early-August and also lasted about a month. The most abundant subspecies was the Southwestern Willow Flycatcher (E. t . extimus), followed by E. t. brewsteri, E. t. adastus, and E. t. trai llii. Nearly half of the Willow Flycatchers captured had no observable fat. Spring flycatchers had more fat stores than fall flycatchers. Wi llow habitat had the highest capture rate among the habitats sampled. Willow Flycatchers caught in willow habitat had higher fat stores than those caught elsewhere. Recaptured Willow Flycatchers had an average body mass gain of 1.6%/day with a short stopover length. Most Willow F lycatchers were unable to reach their destinations in a single flight, making it necessary for them to replenish their energy stores elsewhe re en route. We suggest that the riparian woodlands of the middle Rio Grande provide important refueling sites for stopover flycatchers as t hey migrate between their breeding and wintering grounds.