WSP03308

• File Number: 8220.105.I
• Description: Colorado River-Water Projects-Navajo-Environmental Studies
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/26/2004
• Title: Navajo Dam EIS-Draft Bio Opinion-US Fish and Wildlife Service New Mexico Ecological Services Field Office
• Water Supply Pro - Doc Type: Report/Study

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Area Manager
DRAFT

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catastrophic events occur that would remove or significantly reduce habitat suitability at those
sites.

Additionally, flycatchers no longer occur at 65 of the 221 sites located and/or monitored
rangewide since 1993 and all but two of these sites had less than 5 flycatcher territories present
(Sogge et al. 2002). The two exceptions, PZ Ranch on San Pedro River (1996) and Colorado
River Delta at Lake Mead (1996), were destroyed by fire and lake inundation, respectively.

Reproductive Success. In New Mexico, breeding success has been studied in the Gila
River sites, and along the Rio Grande. In 2001, 133 nests were monitored in the Gila River near
Gila-Cliff Valley, New Mexico. Data indicated that 34.4 percent of the nesting attempts were
successful (Broadhead et al. 2002). Along the Rio Grande in 2002, 80 nests were monitored and
success was 55 percent (Ahlers, in prep.). In 2001, 45 nesting attempts were documented, and 73
percent of these were successful (Ahlers et al. 2002). In 2000, the nest success along the Rio
Grande was 65 percent of26 monitored nests (Ahlers et al. 2001). Nesting was usually initiated
in Mayor early June, with the first eggs documented in the last week of May, and the latest egg
laying in the last week of July (Reclamation 2003).

In 2001, a total of 426 nesting attempts were documented in Arizona at 40 sites (Smith et al.
2002). The outcome from 329 nesting attempts was determined (not every nesting attempt was
monitored). Of the 329 nests monitored, 58 percent (n=191) were successful, 35 percent failed
(n=1 14), and 7 percent (n=24) had an outcome which could not be determined. Causes of nest
failure were predation (n=82), nest desertion (n=IO), brood parasitism (n=6), infertile clutches
(n=12), weather (n=2), and unknown causes (n=2). Cowbirds may have contributed to other
abandoned nests, but no direct evidence was detected. Three parasitized nests fledged
flycatchers along with cowbird young. Nine sites had cowbird trapping in 2001 (Alamo Lake,
Greer/Alpine [Alpine Horse Pasture and Greer River Reservoir], Roosevelt Lake [Lake shore],
and Winkelman [CB Crossing, Cook's Lake, Dudleyville Crossing, Indian Hills, and KearnyJ).

Predation and Cowbird Parasitism. Cowbird parasitism of flycatcher broods has been
documented throughout its range (Brown 1 988a,b; Whitfield 1990; Muiznieks et al. 1994;
Whitfield 1994; Hull and Parkerl995; Maynard 1995; Sferra et al. 1995; Sogge 1995b). Where
studied, high rates of cowbird parasitism have coincided with flycatcher population declines
(Whitfield 1994; Sogge 1995a,c; Whitfield and Strong 1995) or, at a minimum, resulted in
reduced or complete nesting failure at a site for a particular year (Muiznieks et a1.1994; Whitfield
1994; Maynard 1995; Sferra et al. 1995; Sogge 1995a,c; Whitfield and Strong 1995). Cowbird
eggs hatch earlier than those of many passerine hosts, thus giving cowbird nestlings a
competitive advantage (Bent 1960; McGeen 1972; Mayfield I 977a,b; Brittingham and Temple
1983). Flycatchers can attempt to renest, but renesting often results in reduced clutch sizes,
delayed fledging, and reduced nest success (Whitfield 1994). In one study, cowbird parasitism
was often the cause of delayed fledging. Nestlings that fledged later than July 20 had a
significantly lower return rate than those fledging earlier (Whitfield and Strong 1995).

Intensive nest monitoring efforts in California, Arizona, and New Mexico have shown that
cowbird parasitism and/or predation can often result in failure of the nest; reduced fecundity in