Degree of Threat: Very high - high
Comments: The primary threat is the loss and degradation of habitat, particularly riparian forests (Gaines 1974; Gaines and Laymon 1984; Laymon and Halterman 1987b; Hughes 1999; Corman, in Corman and Wise-Gervais 2005).
USFWS (2001) concluded that the decline is primarily attributable to habitat loss, degradation, and fragmentation, resulting from overgrazing, replacement of native riparian woodland species by tamarisk and other non-native plants, and river management, including altered flow and sediment regimes, and flood control practices, such as channelization and bank protection.
HABITAT LOSS and DEGRADATION: Riparian forests have declined throughout the west as a result of conversion to agricultural and other uses, dams and river flow management, stream channelization and stabilization, livestock grazing, groundwater pumping, and invasion of alien vegetation such as tamarisk (USFWS 2003, Hunter et al. 1988, Ehrlich et al. 1992). Estimates of riparian habitat losses include 90-95 percent for Arizona, 90 percent for New Mexico, and 90-99 percent for California (Ohmart 1994; Noss et al. 1995). In California, and undoubtedly elsewhere, the remaining riparian habitat is largely in a degraded condition (Katibah 1984). Livestock grazing is known to negatively affect populations of other riparian birds (Bock et al. 1993), and riparian areas are preferred by livestock over the surrounding xeric uplands (Ames 1977, Valentine et al. 1988, Johnson 1989). In addition, white-tailed deer may have negative consequences for cuckoos in Pennsylvania, yellow-biiled cuckoos were aabsent from an experimental forest where deer density exceeded 7.9 deer per square kilometer (Decalesta 1994). This species is considered to be very vulnerable to deforestation on the wintering grounds (Morton 1992).
HABITAT FRAGMENTATION: Habitat fragmentation is a major threat; in California, nesting by yellow-billed cuckoos may require intact woodlands of at least 40 hectares, and woodlands greater than 80 hectares appear to be preferred (Laymon and Halterman 1989). Along the Sacramento River, California, much of the remaining riparian habitat occurs in narrow, disconnected strips (Halterman et al. 2001), which are not utilized by the cuckoo for nesting (Gaines 1974). Eastern yellow-billed cuckoos, however, can nest in smaller, although still not insubstantial fragments: in a Florida Keys study, for example, they were not present in forest fragments smaller than 7.5 hectares (Bancroft et al. 1995).
ALIEN SPECIES: Significant habitat degradation in the Southwest has been caused by the invasion of tamarisk (Tamarix spp.). Tamarisk changes riparian forests by destroying community structure, replacing three or four vegetation layers with one monotonous layer. Human disturbance of riparian habitats (e.g. damming and flow alteration, catlle grazing) has allowed tamarisk to outcompete native vegetation (USFWS 2003). Tamarisk invasion typically coincides with reduction or loss of bird species associated with cottonwood-willow habitat, including yellow-billed cuckoo (Hunter et al. 1987, Hunter et al. 1988, Rosenberg et al. 1991). However, along the Pecos River in Texas, tamarisk has invaded where no riparian forest existed before, and this has allowed some cuckoos to establish there (Hunter et al. 1988).
PESTICIDES: Significant eggshell thinning and low to moderate levels of DDT and DDE have been detected. Prey scarcity (linked at least in part to pesticide use) may also play a role in declines even where suitable habitat remains, and nestlings may be poisoned if sprayed directly (Laymon and Halterman 1987b).
TOWER STRIKES: Mortality due to tower strikes has been reported (Cohrs 1994).
