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I I I I I I I I I I I I I I I I I ii, I BIOLOGICAL OPINION Status of the Species Whitfield and Strong (1995) found that, besides lowering nest success, fecundity, and the number of young produced, cowbird parasitism may also lower survivorship of flycatcher young fledged late in the season. Southwestern willow flycatchers that abandon parasitized nests, or renest after fledging cowbirds, lay fewer eggs in subsequent clutches and, if successful, fledge flycatcher young late in the season. Whitfield and Strong (1995) determined that cowbird parasitism delayed successful flycatcher nesting by at least 13 days and this delay resulted in significantly different return rates of juveniles. Only 6.4 % of flycatcher young that came from late nests were recaptured in subsequent years, whereas 21.9% of young that came from early nests were recaptured. If these recapture rates mirror actual survivorship, then even though some parasitized flycatchers eventually fledge their own young, nest loss due to parasitism or depredation may have the more insidious effect of reducing overall juvenile survivorship. Despite the cowbird trapping program and increased reproductive success, Whitfield has not observed a population increase at her study area. Whitfield and Strong (1995) speculate that other factors in addition to cowbird parasitism, such as habitat loss, pesticide use on wintering grounds, and stochastic events such as storms resulting in mortality, may be keeping population numbers low. The number of unmated, territorial flycatchers and paired flycatchers detected along the Colorado River in the Grand Canyon has remained low since monitoring began in 1982. Brown (1988) reported that at least 50 % of flycatcher nests monitored in the Grand Canyon between 1982 and 1987 were parasitized by brown-headed cowbirds, but he did not report data on productivity. Since 1992, 10 known pairs of willow flycatchers have made 14 nesting attempts in the Grand Canyon, two of which successfully fledged a total of four flycatchers (Sogge and Tibbitts 1992, Sogge et al. 1993, Sogge and Tibbitts 1994, Sogge et al. 1995). This low rate of reproduction indicates that, even with the protections provided annually by the National Park Service (i.e., camping and other activities are prohibited at flycatcher breeding sites), this area is a population sink (Pulliam 1988) where reproduction is not adequate to replace adults and population persistenc e requires emigration from other breeding areas. Elsewhere in Arizona, population loss or undetected dispersal of breeding groups has been documented since 1993. For example, surveys in 1993 est~ated five territorial males at Dudleyville Crossing on the San Pedro River (Pinal Co.). However, surveys in 1994 and 1995 failed to detect any flycatchers at that location (Muiznieks et al. 1994, Sferra et al. 1995, Spencer et al. 1996). On the Verde River at Clarkdale in Yavapai Co., four pairs of flycatchers were present in 1992. Due to cowbird parasitism and nest predation, there was poor reproductive success at this site each successive year. By 1996, no flycatchers remained, although a new site 2.4 km away, Tavasci Marsh, was found to have one nesting pair. Flycatchers detected in 1993 at Soza Wash on the San Pedro River were not detected in followup surveys in 1995, and a flycatcher observed at Ister Flat on the Verde River was not detected in followup surveys during 1994. It is not known whether these events represent mortality of flycatchers, changes in habitat 57