|
<br />Nesting southwestern willow flycatchers were common in Glen Canyon in the 1950's (Behle and
<br />Higgins 1959). This area was inundated by Lake Powell and no singing male willow flycatchers
<br />were detected in a 1991 survey below Glen Canyon darn, although weather may have been i
<br />factor (Brown 1991). ,Further down river, in the same survey, two pairs and nests were located at
<br />RM 50.7 and at RM 71.1 (Cardenas Marsh). In an earlier 6-year study, Brown (1988) noted a
<br />population increase in the Grand Canyon from two in 1982, to a maximum of 11 (two in Cardenas
<br />Marsh) in 1984, with a subsequent decline to seven in 1981. Only two pair were noted in 1991
<br />(Brown 1991). Surveys in 1992 detected seven willow flycatchers, three unpaired males and two
<br />breeding pairs in Cardenas Marsh (RM 71; Sogge and Tibbitts 1992). Surveys in 1993, 1994 and
<br />1995 revealed two, four and one territorial pairs, respectively. Only one young was fledged in
<br />1995. In 1996 Petterson and Sogge (1996) reported two territorial males, and one successfully
<br />breeding pair along the Colorado River in upper Grand Canyon. The pair at 50.5L fledged one,
<br />and possibly two, young. Nesting success appears to be limited both by brown-headed cowbird
<br />(Molothnts ater) brood parasitism, and possibly by a lack of females in upper Grand Canyon.
<br />
<br />Robert McKernan (San Bernardino County Museum, personal communication) reported seven
<br />active E!. extimus nests on the upper Lake Mead delta in Lake Mead National Recreation Area
<br />but outside of Grand Canyon National Parle boundaries in 1996. However, no nesting
<br />southwestern willow flycatchers have been reported from the Lake Powell headwater deltas.
<br />
<br />Because the Grand Canyon population historically occurred between Colorado River Miles-(RM}
<br />46 and 54 and at RM 71 (Unitt 1987), the U.S. Fish and Wildlife Service designated the Colorado
<br />River from RM 39 to RM 11.5 as critical habitat (U.S. Fish and Wtldlife Service 1993). The
<br />boundary of this area includes the main river channel and "all associated side channels,
<br />backwaters, pools and marshes throughout the May-September breeding season and areas within
<br />100 meters of the edges of the surface water".
<br />
<br />Life Requisites
<br />
<br />Southwestern willow flycatchers arrive in the Grand Canyon area in mid-May, but may be
<br />confused with another subspecies, the more common E.!. brewsteri, which migrates through to
<br />more northern breeding grounds (Aldrich 1951; Unitt 1987). EL brewsteri sings during
<br />migration, making sub-specific distinctions difficult until mid-June (Brown 1991). Males arrive
<br />earlier in spring than do females, and the males establish territories.
<br />
<br />E.!. extimus nests along rivers, streams and wetlands in dense vegetation (Arizona Game and Fish
<br />Department 1996). These areas provide both nesting and foraging habitat. Dense, multistoried
<br />vegetation near surface water or moist soil is consistently selected by breeding birds. In Utah,
<br />E!. extimus was confined to areas of 10-100% shrub density, with few large trees (Whitmore
<br />1917). Structural complexity of riparian vegetation is important and highly correlated with habitat
<br />use (Whitmore 1915). Nesting in the Grand Canyon occurs in thickets of vegetation
<br />approximately 4-1 m tall (13-23 feet), with a dense volume of foliage 0-4 m from the ground
<br />(Tibbetts et al. 1994).
<br />
<br />21
<br />
<br />_n__ i
<br />~
<br />,
<br />
<br />,
<br />~'
<br />'.
<br />
<br />,
<br />i ~
<br />
<br />(
<br />
<br />L^
<br />
<br />::'1
<br />
<br />,
<br />1,"
<br />
<br />,~
<br />"
<br />
<br />~~, I
<br />h,
<br />
<br />.'
<br />,2'
<br />::.-.i
<br />;l!1
<br />!ii
<br />I:~ II
<br />~
<br />'"
<br />~1
<br />
<br />c.'"
<br />
<br />;,.
<br />
<br />.
<br />,,: l':t;'
<br />
<br />i:;
<br />,
<br />r:.'
<br />k. ~~'
<br />~ '.
<br />
<br />(#;
<br />
<br />;k1
<br />,..;
<br />i~,
<br />~.
<br />~1
<br />
<br />,,":;:
<br />
|