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

Territory Size

Southwestern willow flycatcher territory size, as defmed by song locations of territorial birds,
probably changes with population density, habitat quality, and nesting stage. Early in the season,
territorial flycatchers may move several hundred meters between singing locations (Sogge et al.
1995, Petterson and Sogge 1996, R. Marshall pers. obs.). It is not known whether these
movements represent poly territorial behavior or active defense of the entire area encompassed by
singing locations. However, during incubation and nestling phases, territory size, or at least the
activity centers of pairs, can be very small and restricted to an area less than one-half hectare.
Sogge et al. 1995 estimated a breeding territory size of 0.2 ha for a pair of flycatchers occupying
a 0.6 ha patch on the Colorado River in Grand Canyon. Activity centers may expand after young
are fledged but while still dependent on adults.

Reproductive Success

Intensive nest monitoring efforts in California, Arizona, and New Mexico have revealed that: (1)
sites with both relatively large and small numbers of pairs have experienced extremely high rates
of brood parasitism; (2) high levels of cowbird parasitism in combination with nest loss due to
predation have resulted in low reproductive success and, in some cases, population declines; (3)
at some sites, levels of cowbird parasitism remain high across years, while at others parasitism
varies temporally with cowbirds absent in some years; (4) the probability of a flycatcher
successfully fledging its own young from a nest that has been parasitized by cowbirds is low (i.e.,
< 5%); (5) cowbird parasitism and nest loss due to predation often result in reduced fecundity
in subsequent nesting attempts, delayed fledging, and reduced survivorship of late-fledged young;
and, (6) nest loss due to predation appears more constant from year to year and across sites,
generally in the range of 30 to 50 % .

On the South Fork Kern River (Kern Co., CA), Whitfield (1993) documented a precipitous decline
in the flycatcher breeding population from 1989 to 1993 (44 to 27 pairs). During that same period
cowbird parasitism rates between 50% and 80% were also documented (Whitfield 1993) (Table
5).

A cowbird trapping program initiated in 1993 reduced cowbird parasitism rates to < 20%.
Flycatcher population numbers appear to have stabilized at 32 to 34 pairs in 1993, 1994, and 1995
(Whitfield 1994, Whitfield and Strong 1995). Predation rates have remained relatively constant
in the range of 33 to 47% (Table 5). Flycatcher nest success increased from 26% prior to cowbird
trapping to 48% after trapping was implemented (Whitfield and Strong 1995). In addition, the
number of young fledged also increased from 1.01 young/pair to 1.73 young/pair during the same
period.

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