The southwestern willow flycatcher (Empidonax traillii extimus) is a small bird that breeds in dense riparian habitats across the southwestern United States. This subspecies of the willow flycatcher was listed as endangered under the Endangered Species Act in 1995 due to population declines resulting from habitat loss and degradation. Understanding the nesting requirements of the southwestern willow flycatcher is important for conservation efforts aimed at recovering this species.
The southwestern willow flycatcher is a neotropical migrant that breeds in the southwestern U.S. and migrates to Mexico, Central America, and possibly northern South America for the winter. Its breeding habitat consists of dense riparian vegetation near surface water or saturated soil. This species typically places its nest in the fork of a branch of a tree or shrub within this dense riparian habitat. Factors such as vegetation composition, vegetation structure, hydrology, and presence of surface water influence the quality of nesting habitat for the southwestern willow flycatcher.
Vegetation Composition
The southwestern willow flycatcher nests in riparian habitats dominated by native broadleaf trees and shrubs. The species composition of vegetation varies across the subspecies’ range, but frequent nesting substrates include willow species such as Goodding’s willow (Salix gooddingii), boxelder (Acer negundo), tamarisk (Tamarix spp.), Russian olive (Elaeagnus angustifolia), buttonbush (Cephalanthus occidentalis), and cottonwood tree (Populus fremontii). This bird may also nest in other riparian shrubs, trees, or vines, but nests almost exclusively in native vegetation in more pristine habitats.
Tamarisk is an invasive shrub that has proliferated in many southwestern riparian systems where it often forms dense, monotypic thickets. Although tamarisk dominance is associated with degraded conditions, the southwestern willow flycatcher will readily use it for nesting where native vegetation is lacking. Restoration efforts that remove tamarisk without replacing it with suitable native vegetation can negatively impact flycatcher breeding habitat in the short term.
Vegetation Structure
In addition to vegetation composition, the structure of the habitat is a key component of nesting suitability. The southwestern willow flycatcher requires dense riparian habitat, with the highest densities occurring in sites with a high percentage of dense foliage cover within the first 3-7 meters above ground. Canopy cover greater than 50% is typical of suitable nesting sites. Habitats with a dense, contiguous canopy and high foliage volume provide favorable conditions for nest concealment and protection.
Nest sites typically have a dense tree or shrub canopy immediately around the nest. Densities of about 50,000-60,000 stems/hectare have been reported at occupied sites, with nests usually placed in the densest, contiguous patches. Lower stem densities can provide suitable habitat when the foliage density is very high. The bird may place nests along habitat edges, but avoids nesting in small, isolated patches.
Vegetation Height
In addition to a dense canopy, suitable nesting patches contain vegetation of appropriate height. Habitats are often characterized by trees and shrubs ranging from 2-7 meters in height. Nests are typically built in the understory or midstory vegetation, ranging from 0.6 – 6 meters above ground. Extensive overstory canopy above 7 meters may preclude use by the flycatcher. Conversely, habitats where the vegetation is too short, such as sites lacking well-developed midstory vegetation, are also unsuitable.
Hydrology
The southwestern willow flycatcher depends on riverine breeding sites with slow moving or still surface water or saturated soils. Habitats with standing water, slow streams, or boggy saturation tend to have dense vegetation growth suitable for nesting. Such hydrological conditions are key to the development of dense riparian corridors used by the southwestern willow flycatcher.
Nesting is closely associated with surface water or saturated soil conditions. Studies across the subspecies’ range have found surface water within an average of 8 meters of nest sites. The bird typically avoids breeding at sites lacking proximity to water or saturated soils. Loss of surface flows and hydrological modifications that lower water tables or reduce soil moisture can quickly lead to habitat degradation. Managing river flows and groundwater levels is therefore important for maintaining nesting habitat suitability.
Water Sources
Surface water characteristics providing suitable hydrological conditions vary across the subspecies range, but frequently include streams, pools in intermittent channels, backwaters, lake edges, and cienegas. Dammed portions of rivers and streams can provide suitable hydrology. Sources may be perennial, intermittent, or ephemeral.
The bird nests along both natural and artificial waterways, including irrigation canals, reservoir margins, and effluent wetlands. The key factor is the presence of slow moving or still water to provide saturated soil conditions. Flowing water may deter nesting very close to stream banks, but does not preclude use of streamside habitats.
Presence of Surface Water
The proximity of surface water or saturated soils is one of the most consistent distinguishing characteristics of suitable nesting habitat across the subspecies’ range. The majority of nests are found within 50 meters of surface water or saturated soil. Vegetation moisture levels rather than visible surface water may be a key factor driving habitat selection. Surface water creates soil moisture levels supporting dense vegetation growth.
Surface water may provide important foraging habitat. The species is aerial insectivorous, foraging by sallying and hovering to catch insects on the wing. Proximity to open water, stream edges, or boggy soil where emerging aquatic insects are abundant may enhance foraging opportunities. However, moisture levels sufficient to maintain dense riparian vegetation structure appear to be a larger determinant of nesting habitat quality than prey availability.
Geographic Variation
The specific vegetation composition and structure surrounding nests varies across the southwestern willow flycatcher’s range. However, nests consistently occur in dense riparian habitats with moist soil conditions across all breeding regions.
In Arizona, nests are often placed in thickets dominated by tamarisk along tributaries and backwaters of the Gila River. Goodding’s willow and boxelder are other important nesting plants there.
California sites are typified by dense willows and cottonwoods along perennial streams, lakes and oxbows, and ponded ephemeral channels.
Habitats with boxelder, Russian olive, tamarisk, buttonbush and willows predominate in parts of Colorado and New Mexico.
In Texas, the bird nests along broad desert streams supporting a diversity of mesic vegetation including Goodding’s willow, velvet ash, buttonbush, tamarisk, and native brush.
Despite this geographic variation, the unifying feature is the dense riparian vegetation structure associated with moist soil conditions. Maintaining appropriate hydrology and vegetation are therefore key components for conservation.
Threats to Nesting Habitat
Loss and degradation of suitable nesting habitat is a primary threat to the southwestern willow flycatcher. Hydrological changes, inappropriate grazing, recreation impacts, and habitat replacement have contributed to the loss and fragmentation of nesting habitat across the subspecies’ range. Ongoing protection and management of riparian areas is crucial for recovery.
Diversion, damming, and channelization of streams for agriculture and other human uses have greatly altered natural hydrological regimes across the Southwest. These changes dry out floodplains and lower water tables, reducing moisture levels that support dense riparian growth. Sedimentation from livestock grazing and upstream land uses can further alter natural stream functions.
Poorly managed livestock grazing in riparian areas negatively impacts nesting habitat by removing vegetative cover, trampling streambanks, and altering hydrology. Rodents can also degrade flycatcher habitat. Prolonged drought stresses riparian vegetation and reduces surface flows.
Replacement of native riparian communities with exotic species such as tamarisk has substantially altered habitat composition. Although tamarisk provides usable nesting substrate, monotypic stands are an indication of degradation. Tamarisk removal projects in the absence of active native riparian restoration can temporarily eliminate already limited habitat.
Ongoing protection and restoration of riparian systems that mimics natural hydrological regimes will be key for recovering the southwestern willow flycatcher. Management should focus on regaining functioning floodplain processes, restoring native vegetation diversity, and protecting large contiguous riparian habitat patches. Monitoring responses to existing habitat restoration projects will provide important guidance for future efforts. Continued research into specific habitat requirements across the subspecies’ range will help refine appropriate habitat targets. With sufficient suitable habitat, the southwestern willow flycatcher can recover from its currently imperiled status.
Conclusion
The southwestern willow flycatcher depends on dense riparian habitat with specific vegetation characteristics for nesting. Suitable breeding sites contain dense growth of tree and shrub species that occurs under moist soil conditions provided by proximity to surface water or groundwater sources. Although the plant species composition varies across the subspecies’ range, suitable habitat consistently consists of contiguous areas with dense foliage within the first 3-7 meters above ground and surface water typically within 50 meters of nest sites. Ongoing protection and restoration of hydrological processes and vegetation characteristics that create these moist riparian thickets will be essential for the conservation of the southwestern willow flycatcher’s nesting habitat. Maintaining suitable breeding conditions and habitat connectivity across the species’ range will aid the recovery of this endangered bird.
