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<br />BIOLOGICAL OPINION
<br />
<br />Status of the Species
<br />
<br />typically lay three to four eggs per clutch (range = 2-5). The breeding cycle, from laying of the
<br />first egg to fledging, is approximately 28 days. Eggs are laid at one-day intervals (Bent 1960,
<br />Walkinshaw 1966, McCabe 1991); they are incubated by the female for approximately 12 days;
<br />and young fledge approximately 12 to 13 days after hatching (King 1955, Harrison 1979).
<br />Southwestern willow flycatchers typically raise one brood per year but have been documented
<br />raising two broods during one season (Whitfield 1990). Renesting has been documented after nes t
<br />failure (Whitfield 1990, Sogge and Tibbitts 1992, Sogge et al. 1993, Sogge and Tibbitts 1994,
<br />Muiznieks et aI. 1994, Whitfield 1994, Whitfield and Strong 1995).
<br />
<br />Whitfield (Kern River Preserve, pers. comm.), who has accumulated the largest data set on E.t.
<br />extimus, reported the following data on survivorship of adults and young: of 58 nestlings banded
<br />since 1993, 21 (36%) returned to breed; of 57 birds banded as adults (after hatch year) since 1989,
<br />18 (31 %) returned to breed at least one year (10 males, 8 females), five (9%) returned to breed
<br />for two years (all males), and two (3.5 %) returned to breed for three years. Whitfield (1995) also
<br />documented statistically significant variation in return rates of juveniles as a function of fledging
<br />date; approximately 21.9% of juveniles fledged on or before July 20th returned to her study area
<br />the following year, whereas only 6.4% of juveniles fledged after July 20th returned the following
<br />year.
<br />
<br />Walkinshaw (1966), who studied E,t, traillii in Michigan, estimated that 40.9% of the males at his
<br />study site returned to breed for at least two years, 22.7% returned for at least three years, 13.6%
<br />returned for at least four years, and at least 4.5% returned during their fifth year. Female return
<br />rates were substantially lower. Only 22.6% returned to breed for one year. Neither of the
<br />Whitfield nor Walkinshaw incorporate potential emigration rates into their estimates of returns and,
<br />thus, may underestimate actual survivorship. However, these data are consistent with survival
<br />rates for other passerines (Gill 1990, chap. 21) suggesting that the lifespan of most E. t. extimus
<br />is probably two to three years (i.e., most flycatchers survive to breed one or two seasons).
<br />
<br />Brood parasitism of southwestern willow flycatcher nests by the brown-headed cowbird (Molothrus
<br />ater) has been documented throughout the flycatcher's range (Brown 1988, Whitfield 1990,
<br />Muiznieks et al. 1994, Whitfield 1994, Hull and Parker 1995, Maynard 1995, Sferra et al. 1995,
<br />Sogge 1995b). Cowbirds lay their eggs in the nests of other species, directly affecting their hosts
<br />by reducing nest success. Cowbird parasitism reduces host nest success in several ways. Cowbirds
<br />may remove some of the host's eggs, reducing overall fecundity. Hosts may abandon parasitized
<br />nests and attempt to renest, which can result in reduced clutch sizes, delayed fledging, and reduced
<br />overall nesting success and fledgling survivorship (Whitfield 1994, Whitfield and Strong 1995).
<br />Cowbird eggs, which require a shorter incubation period than those of many passerine hosts, hatch
<br />earlier giving cowbird nestlings a competitive advantage over the host I s young for parental care
<br />(Bent 1960, McGeen 1972, Mayfield 1977, Brittingham and Temple 1983). Where studied, high
<br />rates of cowbird parasitism have coincided with southwestern willow flycatcher population decline s
<br />(Whitfield 1994, Sogge 1995a, Sogge 1995c, Whitfielq and Strong 1995), or resulted in reduced
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