THE SOUTHWESTERN WILLOW FLYCATCHER AN ENDANGERED SPECIES IN THE COLORADO RIVER DELTA VANDA J. GERHART ENVIRONMENTAL RESEARCH LABORATORY UNIVERSITY OF ARIZONA

Southwestern Willow Flycatcher (Empidonax traillii extimus) Neotropical migratory bird Placed on endangered species list by USFWS 1997.Critical habitat designation enacted. 18 critical habitat units totaling 599 river miles in AZ, CA, & NM.

Description Small passerine bird. 15cm long, weight ~12 g Greenish -brownish gray above White throat Pale olive breast Pale yellow belly 2 white wing bars present Eye-ring faint to absent Upper mandible dark, lower light. Distinguishing feature is its song. The Fitz-bew and Whitt.

Geographic Range of the Willow Flycatcher Wintering habitats: Known: Costa Rica, Belize. Probable: Central America, N. South America, Southern Mexico. Migration Routes: Unknown Breeding Grounds: S.W. United States, N. Sonora, N. Baja

Breeding Range and Distribution of Willow Flycatcher Subspecies. Adapted from Unitt 1987 and Browning 1993

HABITAT Dense riparian habitat from sea level to 2600 m. < 18 m from water or saturated soil. Plant species composition: monotypic to mixed species. Habitat structure: Live foliage density high from the ground to the canopy. Nesting sites located in riparian habitats wider than 10m.

THREATS TO CRITICAL BREEDING GROUNDS OF THE SOUTHWESTERN WILLOW FLYCATCHER Causes: Urban encroachment Recreation & Agriculture Fire Water diversion Water impoundment Channelization Livestock & grazing Exotic vegetation EXTENSIVE LOSS, FRAGMENTATION & MODIFICATION OF RIPARIAN AREAS

NESTING HABITS Nest height variable: range 2 to 7 m above ground. Nests: 8 cm h x 8 cm w. Nests typically in fork of branch. Clutch size: 3-4 eggs. Incubation:12-13 days. Fledge days. Stay close to nest 3-5 days Remain in natal area days. Brood parasitism by brown-headed cowbird (Molothrus ater).

CURRENT POPULATION ESTIMATES 300 to 500 pairs 75% of locations there are 5 or less territorial birds. 20% of locations there are single, unmated birds.