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

or complete elimination of nesting success (Muiznieks et al. 1994, Whitfield 1994, Maynard 1995,
Sferra et al. 1995, Sogge 1995a, Sogge 1995c, Whitfield and Strong 1995). Whitfield and Strong
(1995) found that flycatcher nestlings fledged after July 20th had a significantly lower rate of
survival, and that cowbird parasitism was often the cause of delayed fledging.

Habitat Use

The southwestern willow flycatcher occurs in dense riparian habitats from sea level in California
to over 7000 feet in Arizona and southwestern Colorado. Throughout its wide geographic and
elevation range, its riparian habitat can be broadly classified into five types based on plant species
composition and habitat structure (Table 3).

Table 3. Generalized riparian habitat types occupied by the southwestern willow flycatcher
based on plant species composition and vegetation structure1.

Plant Species Composition

Vegetation Structure

Monotypic
Native

Multiple
Species
Native

Multiple
Species
Native/
Non-Native
Mix

Monotypic
Non-Native

Single Stratum
No Distinct Canopy

I

II

Multiple Strata
Distinct Canopy &
Sub-canopies

III

IV

V

1 Roman numerals reference descriptive narrative in text.

The matrix in Table 3 provides a framework for understanding the most conspicuous attributes of
flycatcher habitat, but not necessarily the most important. These attributes are identifiable from

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