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""!"~ !',,\\ ~"'/-;'el f~~ {!;: Sogge et al. 1995). SWWF habitat loss in Central and South America has also undoubtedly contributed to recent SWWF population declines. :!? of; t, , ,;'< '~~. The SWWF has been extirpated from much of its former range (Hunter et al., 1987) and experienced such a sharp reduction in abundance since 1950 that it was initially proposed (U.S. Fish and wildlife Service 1992) and listed, with critical habitat, under the Endangered Species Act, on July 23, 1993 (U.S. Fish and Wildlife Service 1993). The SWWF is more rare than most other currently listed avian species (Unitt 1987). An estimated 300-500 breeding pairs remain in the United States, including 115 pairs in California and approximately 100 pairs in New Mexico (U.S. Fish and wildlife Service 1993). Limited information exists for Colorado, Utah, Nevada, and Texas. It has been given endangered species status by the Game and Fish Departments in Arizona, New Mexico and California. f ~'~:' ~.:~~ Arizona has experienced the sharpest decline in SWWF numbers. The SWWF formerly bred throughout the state at high and low elevations. For example, a 1931 breeding record exists from the south rim of the Grand Canyon (Brown et al., 1984). The former range of SWWF included the lower Colorado River, from which it has been extirpated. By 1987, the State population was estimated at less than 25 pairs (Unitt 1987), but much habitat was not surveyed. At least 52 territories or active nests were'reported during extensive surveys in 1993 in Arizona (Muiznieks et al. 1994), and at least 62 active nests were located during a more thorough inventory in 1994 (Sferra et al., 1995). k~t ;~}~; '..' 1',:~" -, ,!,,~ ." ~ ~~ r..h n, t:'O', ",~< The Grand Canyon population occurs between Colorado River Miles 47 and 54, and at River Mile 71 (Unitt 1987, Sogge et al., 1995). In its recent proposal the U.S. Fish and Wildlife Service (1993) designated the Colorado River from River Mile 39 to River Mile 71.5 as critical ,habitat. The boundary of this area includes the main river channel and associated side channels, backwaters, pools and marshes throughout the May-September breeding season, as well as areas within 100 meters of the edges of the surface water. Nesting SWWF were common in Glen Canyon in the 1950's (Behle and Higgins 1959). This area was inundated by Lake Powell and no singing male SWWF were detected in a 1991 survey below Glen Canyon dam, although weather may have been a factor (Brown 1991a). Further down river, in the same survey, two pairs and nests were located at River Mile 50.7 and at River Mile 71.1 (Cardenas Marsh). In an earlier six-year study, Brown (1988) noted a brief population increase in the Grand Canyon from two in 1982, to a maximum of 11 (two in Cardenas Marsh), with a subsequent decline to seven in 1987. only two pairs were noted in 1991 (Brown 1991b). Surveys in 1992 detected seven SWWF, three unpaired males and two breeding pairs in Cardenas Marsh (Sogge et al., 1995a). A total of five SWWF were detected in Grand Canyon in 1995, three territorial but non-breeding males and one breeding pair that fledged a single young (Sogge et al., 1995a). Male SWWF established territories between Colorado River miles 50.5 and 65.3, and the breeding pair nested at mile 50.5. "",<-. ~l ~ ~:~t~; ~~!~ 'i;/': ~ ~~ (f! ~\y ;:);;" :~~~: SWWF arrive in the Grand Canyon area in mid-May, but may be confused with another subspecies, the more common E t brew~r.Ari, which migrates through to more northern breeding grounds (AldriCh 1951; Unitt 1987). E t brewsteri sings during migration, making sub-specific distinctions difficult until mid- June (Brown 1991b). Males arrive earlier ,than females and set up territories. 16