8031

• UCREFRP Catalog Number: 8031
• Author: U.S. Department of the Interior.
• Title: Fish and Wildlife Service Final Biological and Conference Opinion on Lower Colorado River Operations and Maintenance - Lake Mead to Southerly International Boundary.
• USFW Year: 1997.
• USFW - Doc Type: \
• Copyright Material: NO

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BIOLOGICAL OPINION

Status of the Species

photographs or during site visits. There are other important dimensions or characteristics of
southwestern willow flycatcher habitat, including: size, shape, and distribution of vegetation
patches; hydrology; prey types and abundance; parasites; predators; environmental factors (e.g.
temperature, humidity); and interspecific competition. Other factors related to population
dynamics, such as demography (i.e. birth and death rates, age-specific fecundity), the distribution
of breeding groups across the landscape, flycatcher dispersal patterns, migration routes, site
fidelity, philopatry, and degree of conspecific sociality (e. g. colonialness) generally are not well
understood for the southwestern willow flycatcher. However, some of these factors may be critica I
to population dynamics and habitat use. For example, characterizations of suitable breeding habitat
may be significantly biased if observed patterns of habitat use are infl uenced by intrinsic dispersal
patterns and capabilities rather than habitat quality.

The ultimate measure of habitat suitability is a combination of reproductive success and
survivorship that results in a positive rate of population growth. Without long-term data that
correlate or experimentally verify which of the combinations of the above attributes contribute to
population growth, habitat descriptions should be viewed broadly. For example, the matrix in
Table 3, combined with photographic comparisons and quantitative and qualitative descriptions
could be used to characterize "suitable survey habitat," a limited set of characteristics that describe
areas where recent breeding attempts have been documented. Below, descriptions are given that
correspond to the habitat classes in Table 3.

Rangewide, occupied habitat for the southwestern willow flycatcher can be characterized by dense
patches of riparian shrubs or trees including the following general habitat types:

I) monotypic, dense stands of willow (often S. exigua or S. geyeriana above 7000 feet in AZ)
9 to 20 feet in height with no distinct overstory; difficult to penetrate; vertical foliage
density uniformly high (>60%) from ground to canopy (Spencer et al. 1996) (Type I Table
3).

II) monotypic, dense stands of saltcedar 12 to 35 feet in height forming a nearly continuous,
closed canopy (i.e. no distinct overstory); vertical foliage density increases with height;
canopy density uniformly high (approx. 90%); difficult to penetrate (Spencer et at. 1996)
(Type II Table 3).

III) dense stands of mostly Goodding's willow 12 to 40 feet in height characterized by trees of
different size classes, a distinct overstory, subcanopy strata, fallen but li ving trees creating
dense tangles difficult to penetrate (Type IV Table 3).

IV) dense mixtures of native broadleaf trees and shrubs including cottonwood, willows,
boxelder, ash, alder, buttonbush, and stinging nettIe, characterized by a distinct overs tory

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