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<br />Southwescern willow flycatchers begin nesting in late May and early June and fledge young from
<br />late June through mid-August (Willard 1912, Ligon 1961. Brown 1988, Whitfield 1990, Sogge
<br />and Tibbitts 1992, Sogge et ai. 1993, Muiznieks et al. 1994, Whitfield 1994, Maynard 1995).
<br />Southwestern willow flycatchers typically lay 3 to 4 eggs in a clutch (range = 2-5). The
<br />breeding cycle, from laying of the first egg to fledgling, is approximately 28 days. Eggs are
<br />laid at one-day intervals (Bent 1963, Walkinshaw 1966, McCabe 1991); they are incubated by
<br />the female for approximately 12 days; and young fledge approximately 12 to 13 days after
<br />hatching (King 1955, Harrison 1979). Southwestern willow flycatchers typically raise one brood
<br />per year but have been documented raising two broods during one season (Whitfield 1990).
<br />Southwestern willow flycatchers have also been documented renesting after nest failure
<br />(Whitfield 1990, Sogge and Tibbitts 1992, Sogge et al. 1993, Sogge and Tibbitts 1994,
<br />Muiznieks et al. 1994, Whitfield 1994, Whtifield and Strong 1995).
<br />
<br />Whitfield, who has accumulated the largest datasec on E.t. atimus, reported the following data
<br />on survivorship of adults and young: of 58 nestlings banded since 1993,21 (36 percent) returned
<br />to breed; of 57 birds banded as adults (after hatch year) since 1989, 18 (31 percent) returned to
<br />breed at least one year (10 males, 8 females), five (9 percent) returned to breed for two years
<br />(all males), and two (3.5 percent) returned co breed for three years (M. Whitfield, Kern River
<br />Preserve, personal communication) Whitfield (1995) also documented statistically significant.
<br />variation in return rates of juveniles as a function of fledgling date; approximately 21.9 percent '
<br />of juveniles fledged on or before July 20th returned co her study area the following year,
<br />whereas only 6.4 percent of juveniles fledged after July 20th returned the following year.
<br />
<br />Walkinshaw (1966), who studied E.t. traillii in Michigan, estimated that 40.9 percent of the
<br />males at his study site returned to breed for two years, 22.7 percent returned for three years,
<br />13.6 percent returned for four years, and 4.5 percent returned during their fifth year. Females
<br />return rates were substantially lower. Only 22.6 percenc returned to breed for one year.' These
<br />data are consistent with survival rates for other passerines (Gill 1990, chap. 21) and suggest that
<br />the lifespan of most E.t. extimus probably is cwo to three years.
<br />
<br />The southwestern willow flycatcher is a frequent host of the brown-headed cowbird (Molothrus
<br />ater) (Muiznieks et al. 1994, Whitfield 1994, Sferra et al. 1995, Sogge 1995b). Cowbirds lay
<br />their eggs in the nests of other species directly affecting their hosts by reducing nest success,
<br />Cowbird parasitism reduces host nest success in several ways. Cowbirds may remove some of
<br />the host's eggs, reducing overallfecundity. Hosts may abandon parasitized nests and attempt
<br />to renest, which can result in reduced clutch sizes. delayed fledgling, and reduced overall nesting
<br />success and fledgling survivorship (Whitfield 1994). Cowbird eggs, which require a shoner
<br />incubation period than those of many passerine hosts, hatch earlier giving cowbird nestlings a
<br />competitive advantage over the host's young for parental care (Bent 1963, McGeen 1972.
<br />Mayfield 1977. Brittingham and Temple 1983). Where studied, high rates of cowbird parasitism
<br />have coincided with southwestern willow flycaccher population declines, or, at a minimum.
<br />resulted in reduced or complete elimination of nesting success (Muiznieks et al. 1994, Whitfield
<br />1994, 1995, Sferra et al. 1995, Sogge 1995b). Whitfield (1995) found that flycatcher nestlings
<br />fledged after July 20th had a signifICantly lower rare of survival, and that cowbird parasitism
<br />was often the cause of delayed fledgling.
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<br />Biological and Conference Opinions Glen Canyon Beach/Habilal'Bulldmg Flows 2/16196
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