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

Rangewide Present Status

Rangewide, the current known population of southwestern willow flycatchers stands at
approximately 454 territories (Table 6). These results indicate a critical population status; more
than 75 % of the locations where flycatchers have been found are comprised of five or fewer
territorial birds and up to 20% of the locations are comprised of single, unmated individ uals. The
distribution of breeding groups is highly fragmented, with groups often separated by considerable
distances (e.g., approximately 88 km straight-line distance between breeding flycatchers at
Roosevelt Lake, Gila Co., AZ, and the next closest breeding groups known on either the San Pedro
River, Pinal Co., or Verde River, Yavapai Co.). Additional survey efforts, particularly in
southern California, may discover additional small breeding groups. However, rangewide survey
efforts have yielded positive results in less than 10% of surveyed locations. Moreover, survey
results reveal a consistent pattern rangewide: the southwestern willow flycatcher population as a
whole is comprised of extremely small, widely-separated, breeding groups or unmated flycatchers.

The data presented in Table 6 represents a composite of surveys conducted since 1992. Locations
that had flycatchers for only one year were tabulated as if the location is still extant. Given that
extirpation has been documented at several locations during the survey period, this method of
analysis introduces a bias that may overestimate the number of breeding groups and overall
population size. In addition, females have been documented singing as frequently as males.
Because the established survey method relies on singing birds as the entity defining a territory
(Tibbitts et al. 1994), double-counting may be another source of sampling error that biases
population estimates upward. The figure of 454 southwestern willow flycatcher territories is an
approximation based on considerable survey effort, both extensive and intensive. Given sampling
errors that may bias population estimates positively or negatively (e.g., incomplete survey effort,
double-counting males/females, composite tabulation methodology), natural population fluctuation,
and random events, it is likely that the total population of E.t, extimus is fluctuating between 300
and 500 territories with a substantial proportion of indi viduals remaining unmated. This figure is
alarming because, at such low population levels, random demographic, environmental, and genetic
events could lead to extirpation of breeding groups and eventually render this species extinct, eve n
if all extant sites were fully protected. The high proportion of unmated individuals documented
during recent survey efforts suggests the southwestern willow flycatcher may already be subject
to a combination of these factors (e.g., uneven sex ratios and low probability of finding mates in
a highly fragmented landscape). A recovery plan for the southwestern willow flycatcher has not
yet been prepared.

Effects to Species' Status from Past Consultations

Federal actions that have undergone formal section 7 consultation have also affected the status of
the species rangewide. To date, the Service has completed or begun at least 21 consultations for
the southwestern willow flycatcher (Table 7). Eleven of the 15 completed or draft biological

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