Ebook A Quantitative Analysis of the Diet of Southwestern Willow Flycatchers in the Gila Valley, New Mexico
The Southwestern Willow Flycatcher (Empidonax traillii extimus) is a riparian obligate inhabiting dense stream side thickets and woodland (Sedgwick 2000, Sogge and Marshall 2000). In the past century, most of the riparian habitat in the Southwest has been destroyed or degraded due to urban and agricultural development, water management, channelization, overgrazing, recreation, and invasion by exotic saltcedar (Tamarix ramosissima) (Patten 1998, Cartron et al. 2000, Marshall and Stoleson 2000). The Southwestern Willow Flycatcher has shown a concomitant decline (Unitt 1987), resulting in it being listed as an endangered species in 1995 (U.S. Fish and Wildlife Service 1995). The most recent population estimates total 915 known territories rangewide (U.S. Fish and Wildlife Service 2001).
Although recent research has shed light on various aspects of Willow Flycatcher biology and habitat associations (see Finch and Stoleson 2000, U.S. Fish and Wildlife Service 2001), its food habits remain only poorly known. Previous information on diet has been cursory (Beal 1912, Bent 1942, and McCabe 1991). To date, two descriptive diet studies have been conducted on the southwestern subspecies at several sites in California, Arizona and Colorado by the USGS Colorado Plateau Field Station (Drost et al. 1998, 2001). Based on analysis of fecal samples, those studies documented a wide variety of arthropod prey including both aquatic and terrestrial taxa. This variety of prey items suggests the Willow Flycatcher may be considered a generalist insectivore, but that characterization cannot be made without an understanding of prey availability. Whether or not the Willow Flycatcher is indeed a generalist or whether it specializes in particular prey has important implications for management, especially since observed diets varied among habitat types (Drost et al. 1998) and among sites (Drost 2001).
The largest breeding population of Willow Flycatchers in the Southwest is found in the Cliff Gila Valley of southwestern New Mexico. The habitat used by breeding flycatchers at this mid- elevation site differs in a number of ways from elsewhere in the Southwest: the birds are concentrated in tall, mature riparian forests with box elder (Acer negundo) as a major component (Stoleson and Finch in review). Within this area, the density of flycatchers varies considerably among habitat patches. Proximity to water appears to be correlated with flycatcher abundance, although the underlying mechanism for this relation is unknown. Water may be required for essential prey items.
Here we describe the diet of the Southwestern Willow Flycatcher at this important breeding site. We also compare flycatcher diet to the composition of arthropods as sampled by sticky traps to test the hypotheses that flycatcher diet reflects the relative abundance of different prey taxa (i.e., the bird is a generalist), and flycatcher breeding densities are correlated with arthropod abundance and/or diversity within habitat patches. Finally, we compare flycatcher diet in the Cliff-Gila Valley to that reported from Roosevelt Lake in Arizona and the Kern River in California (Drost et al. 2001).
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