Willow flycatcher (Empidonax traillii) surveys in the Colorado River delta: implications for management

A subspecies of willow flycatcher, the south-western willow flycatcher, has become endangered in the U.S. The objective of this study was to determine the presence/absence of this subspecies in the Colorado River delta. Surveys were conducted in June–July 1999 and in May–June 2000. We detected a total of 50 birds, most likely south-western willow flycatchers, from May to June and none in July. It appears that the birds found in the delta were migrants. It is important to restore the intensively used stopover sites for the recovery of the subspecies. Additionally, we postulate a migratory route throughout the estuaries of Sonora.