Historical and Current Status of the Western Yellow-Billed Cuckoo Excerpted from October 2013 proposed listing rule and updated by the YBCU Core Team

Historical and Current Status Populations of the western yellow-billed cuckoo are too small and isolated in inaccessible habitat patches to be effectively sampled or analyzed for trends by the USGS Breeding Bird Survey (BBS) program, which is conducted by point-counts along roads. In the eastern United States and Canada, where BBS data can be used to analyze yellow-billed populations, these populations have declined by 59 to 67 percent over the past 43 years (USGS 2012). This decline has been linked to both the North Atlantic Oscillation and the El Niño Southern Oscillation, as well as to rising local temperatures (Anders and Post 2006, pp. 221–227). For the western yellow-billed cuckoo, only information from regional and local sources is available to determine population trends, although Clark et al. (2014, p. 147) suggest that long-term weather trends may affect patterns of western yellow-billed cuckoo abundance in southern California.

Pacific Northwest In the Pacific Northwest, including Oregon, Washington, and British Columbia, Canada, the western yellow-billed cuckoo was formerly fairly common locally in cottonwood and willow bottoms along the Willamette and lower Columbia Rivers in Oregon and Washington, and in the Puget Sound lowlands of Washington (Jewett et al. 1953, pp. 342–343; Gabrielson and Jewett 1970, pp. 329–330; Roberson 1980, pp. 225–226; Marshall 1996, pp. 1–2; Marshall et al. 2003, p. 306). They were also found locally in southwestern British Columbia (Hughes 1999, p. 4), but the available data are not adequate to determine historical abundance. Records of western yellow-billed cuckoos were rare east of the Cascade Mountains in these States and Province (Campbell et al. 1990, p. 481; Marshall et al. 2003, p. 306; Wahl et al. 2005, p. 210).

In Oregon, the last confirmed breeding records are from the 1940s. Historically, western yellow- billed cuckoo were considered rare in the State, both in the Willamette Valley, along the lower Columbia River, and in eastern Oregon along the Snake River, although they were fairly common along the Columbia River from 1923 to 1925 (Gabrielson and Jewett 1970, pp. 329– 330). Between 1970 and 1977, four western yellow-billed cuckoo sightings (also referred to as observations or incidental detections) were made west of the Cascade Mountains in the Willamette Valley (Gilligan 1994, pp. 162–163). Between 1970 and 1994 at least 20 western yellow-billed cuckoos have been sighted east of the Cascade Mountains (Gilligan 1994, pp. 162– 163). A 1988 survey in eastern Oregon and Klamath County located no western yellow-billed cuckoos, but identified potential breeding habitat along the lower Owyhee River (Littlefield 1988, p. 34). Recent records from 1990 to 2009 are primarily from May and June and from the east side of the Cascades in Deschutes, Malheur, and Harney Counties (Johnson and O’Neil 2001, pp. 460–461; Cornell Lab of Ornithology 2012). Western yellow-billed cuckoos were

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previously considered a rare annual visitor in Harney County at isolated groves of trees known as vagrant traps and the Malheur National Wildlife Refuge (NWR) (Altman 2001 pers. comm.), but in the last decade it has not been a regular visitor (Marshall et al. 2003, p. 306).

Recent records from the west side of the Cascades at the Sandy River Delta near its confluence with the Columbia River in July of 2009, 2010, and 2012 (Withgott 2012, in. litt.; Leal 2012, in. litt.) were the first observations of the species west of the Cascades since 1977. In June 2010 during surveys on the Columbia River a possible western yellow-billed cuckoo response was heard at Wallace Island, Columbia County, but the sighting could not be verified (Flotlin 2011). Up to 87 percent of wetland and riparian habitat have been lost in the Willamette Valley due to agricultural practices and urbanization (Roth et al. 2004, p. 7). The available data suggest that if western yellow-billed cuckoos still breed in Oregon the numbers are extremely low, with pairs numbering in the single digits.

In Washington, the last confirmed breeding record of western yellow-billed cuckoos was from Seattle in 1923 although there is inconclusive evidence that breeding may have continued into the 1930s. It is likely that western yellow-billed cuckoos have been extirpated as a breeder in the State, although recent observations indicate that the possibility of breeding in the state cannot be ruled out. Of the 24 records between 1836 and 1940 (9 egg sets, 7 specimens, and 8 sight records), 23 were found west and one east of the Cascades. Since 1950, 21 sightings have been documented throughout the State. The most recent sightings include two separate observations on the Little Pend Oreille NWR (in 2012 and 2017) and one in 2015 (Okanogan County). In contrast to historical records, 17 of 21 of the sightings post-1950 were east of the Cascades. Most of these sightings have been incidental as there have not been any standardized surveys in the state. In 2005, Wahl et al. (p. 210) noted that the possibility of a vestigial breeding population in Washington still exists, an idea that is supported by continued incidental sightings despite the lack of any standardized surveys. There are few remaining examples, none of which are extensive, of the river floodplain habitats bordering Puget Sound, which historically had the most western yellow-billed cuckoo sightings in the State (King County 2007, p. 2). The available data suggest that if western yellow-billed cuckoos still breed in Washington, the numbers are extremely low, with pairs numbering in the single digits. In 2017 the Washington Department of Fish and Wildlife recommended listing the species as a state endangered species in Washington (Wiles and Kalasz 2017).

Western yellow-billed cuckoos historically occurred in southwest British Columbia, Canada, in the vicinity of Victoria on Vancouver Island and along the Fraser River system from Vancouver upstream to Kamloops (Bent 1940, p. 64; Campbell et al. 1990, p. 481). The species was apparently never common, with 23 records (18 specimen and 5 sight records) between 1881 and 1927. Two of these observations were of pairs believed to be nesting. The species has been recorded five times in British Columbia since the 1920s, with four of those records from the

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eastern half of the Province where historically the species had not been observed (Campbell et al 1990, p. 481; Siddle 1992, p. 1169; Cornell Lab of Ornithology 2012). As mentioned previously, the species is considered as an extirpated breeder in the Province and is still very rare based on reported observations (British Columbia Conservation Data Centre 2013).

Montana There is very limited data for western yellow-billed cuckoos from the area west of the Continental Divide in Montana. Three specimens have been collected since the early 1960s, and there are few recorded sightings since the early 1900s (Saunders 1921, p. 174). A few records indicate that western yellow-billed cuckoos occurred around the Flathead River area, but there are no confirmed breeding records (Lenard 2001, pp. 1–3). Potential habitat within the range of the western yellow-billed cuckoo in Montana is very limited, and it is unlikely that a breeding population exists within the State.

Idaho In Idaho, the western yellow-billed cuckoo is considered a rare visitor and local summer resident that occurs in scattered drainages, primarily in the southeastern portion of the State (Burleigh 1972, p. 159; Idaho Fish and Game 2005, pp. 222–223; Cavallaro 2011, entire). In northern and central Idaho, there were only four records of western yellow-billed cuckoos during the 20th century (Taylor 2000, p. 252). Reynolds and Hinckley (2005, p. 5) concluded that the few sightings in northern Idaho are most likely of transient, nomadic, or migrant individuals; with no data suggesting that the species historically or currently nests there. In southwestern Idaho the western yellow-billed cuckoo has historically been considered a rare summer visitor and breeder in the Snake River Valley (Idaho Fish and Game 2005, p. 223).

Recent records are primarily from the southeastern portion of the State along the South Fork of the Snake River (Stephens and Sturts 1997, p. 36; Taylor 2000, pp. 252–254; Reynolds and Hinckley 2005, p. 7; Cavallaro 2011, entire). Taylor (2000, pp. 252–254), in his 2000 review of the status of the species in Idaho, concluded that they had declined greatly as a breeding bird in the State, and that there were currently fewer than a few dozen breeding pairs and possibly fewer than 10. More recent surveys of western yellow-billed cuckoos continue to show the majority of sightings are in the Snake River corridor in southeast Idaho with few or no sightings in other areas where the western yellow-billed cuckoo had been historically observed (Reynolds and Hinckley 2005, p. 7; Cavallaro 2011, p. 3). In addition, western yellow-billed cuckoos likely nested in south-central Idaho near Stanton Crossing, Blaine County, in 2003 and 2004 (Reynolds and Hinckley 2005, p. 7). A survey in 2009 near Magic Lake on the Big Wood River located a singing male in a location that was previously unknown (Carlisle and Ware 2010, p. 4). Follow- up surveys in 2010 along the Big Wood River and Little Wood River failed to detect any western yellow-billed cuckoos (Carlisle and Ware 2010, p. 12). However, observations of western yellow-billed cuckoo were made in this area in 2015, 2017, and during formal surveys in 2018

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(Regan and Carlisle 2018, pp. 6–15). The most recent statewide assessment estimated the breeding population in Idaho is likely limited to no more than 10 to 20 breeding pairs in the Snake River Basin (Reynolds and Hinckley 2005, p. 7).

Wyoming Historically, yellow-billed cuckoos were rare and local in Wyoming. Knight (1902, p. 86), in his summary of the birds of Wyoming, did not include the species on the State’s list, and Grave and Walker (1913, p. 46) reported only one record for the State. Prior to 2001, the distribution of yellow-billed cuckoos from summer records of the Wyoming Natural Heritage Database showed a few scattered sightings, with only 12 records from southwestern Wyoming (Bennett and Keinath 2001, pp. 9, 17). Currently, western yellow-billed cuckoo occurs on the western side of the Rocky Mountains along the Lower Green River Basin from the Seedskadee NWR to the Flaming Gorge Reservoir and west to the Bear River Drainage. Within the range of the DPS defined in this document, breeding activity is unconfirmed in Wyoming, but observations suggest that nesting may occur within the Green River Basin and along the Snake River within the State (Deibert 2001, pers. comm., pp. 1–16). On July 4, 2003 a western yellow-billed cuckoo was found by the Wyoming Game and Fish Department in the town of Green River after it collided with a window of their office building (Wyoming Natural Diversity Database 2003 (WYNDD)). In July 2003, yellow-billed cuckoo surveys were conducted at the Seedskadee NWR and on July 10, 2003, a western yellow-billed cuckoo near Big Island in Seedskadee NWR responded with ‘kowlp’ calls to a recorded play-back call (Sweanor pers comm., WYNDD 2003). Call-back surveys were again conducted near Big Island in 2004 by Service personnel. Subsequently, one observation was made of a western yellow-billed cuckoo in 2005 and three western yellow-billed cuckoos were observed in 2006 near Big Island, Seedskadee NWR (Seedskadee NWR, unpublished reports). No other recent surveys have been done (Beason 2010, pp. 2–3). The available literature suggests that the breeding population of the western yellow- billed cuckoo within the State is extremely low, numbering in the single digits, and potential nesting habitat is very limited. Therefore, we conclude that the western yellow-billed cuckoo occurs in very small numbers as a breeder in Wyoming, with likely fewer than five breeding pairs.

Colorado West of the Continental Divide in Colorado, the western yellow-billed cuckoo was probably never common (Bailey and Niedrach 1965, pp. 404–406), and it is now extremely rare (Kingery 1998, pp. 204–205). Western yellow-billed cuckoos were found along the Colorado River in Palisade, near Grand Junction (Mesa County), annually through the 1950s and 1960s (Righter et al. 2004, p. 82). Western yellow-billed cuckoos were also regularly detected as recently as the mid-1980s along the Uncompahgre and Gunnison Rivers near Delta (Delta County) (Beason 2010, p. 1).

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In 1998, the Colorado Breeding Bird Atlas (Kingery 1998, pp. 204–205) gave the general status of the western yellow-billed cuckoo in Colorado as nearly extirpated in the western half of the State. During the 1987 to 1994 period covered by the Atlas, only three western yellow-billed cuckoos were recorded on the western slope, with one confirmed nesting observation along the Yampa River near Hayden in 1988. Other confirmed nesting records (mid-1980s) were associated with outbreaks of caterpillars in box elders in the Four Corners region and Durango area (Colyer 2001, pp. 1–6). National Park Service surveys in southwest Colorado from 1988 through 1995 for the Colorado Bird Breeding Atlas provided no records of western yellow-billed cuckoos.

In 1998, biologists conducted focused western yellow-billed cuckoo surveys along 242 mi (389 km) of lowland river riparian habitat along six rivers in west-central Colorado. They found one probable nesting pair (Dexter 1998, p. 3). Reports of single western yellow-billed cuckoos have come primarily from the Grand Junction area and Mesa County in 2001, 2002, 2005, 2008, and 2011, with a report of more than one western yellow-billed cuckoo at Orchard Mesa Wildlife Area in 2006 (Beason 2010, p. 1; Beason 2012, p. 5). Additional reports include one western yellow-billed cuckoo south of Montrose in Montrose County near the Uncompahgre River in 2009, one western yellow-billed cuckoo along the Gunnison River near Gunnison in 2007 (Beason 2010, p. 1), and detections by the Rocky Mountain Bird Observatory along the Yampa River near Craig in 2007 and 2008 and in far western Colorado near Nucla in 2005 and 2008 (Beason 2010, p. 1). Surveys repeated near Craig and Nucla in 2009 failed to detect western yellow-billed cuckoos. However, the Nucla birds were sighted in 2012, 2015, and 2016 suggesting one or two reoccurring resident cuckoos (eBird, viewed June 27, 2019). Since 2003, western yellow-billed cuckoos have been detected annually at the North Fork of the Gunnison River Valley of west-central Colorado in Delta County, and breeding was confirmed in 2008 and again in 2011 near Hotchkiss (Beason 2010, p. 1; Beason 2012, p. 5).

Western yellow-billed cuckoos have been detected annually since 2001 in the San Luis Valley of south-central Colorado in Conejos County where breeding is suspected, but not confirmed (Beason 2010, p. 1). Surveys conducted on the Rio Grande near Del Norte, Rio Grande County, in 2008 and 2011 found western yellow-billed cuckoos at several locations (Wildlife Specialties, LLC, 2008; Rawinski 2011). Surveys by the Rocky Mountain Bird Observatory in 2010 were conducted near historical detections and at sites with suitable habitat in Archuleta, Conejos, Montezuma, and Rio Grande Counties in south-central and southwest Colorado; no western yellow-billed cuckoos were detected (Beason 2010, p. 2). However, individual cuckoos have been sighted in July 2015 and July 2018 along McElmo Creek in Montezuma County suggesting frequent occurrence during the resident period (eBird, viewed June 27, 2019). Nonetheless, survey results and the available literature indicate a small breeding population of western yellow- billed cuckoos in western Colorado. Therefore, based on the best available information, we conclude that the population of breeding pairs number in the single digits in the State.

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Utah Historically western yellow-billed cuckoos were uncommon in Utah in woodlands along streams in the lower valleys, especially the Salt Lake Valley (Hayward et al. 1976, p. 107). There are scattered records for the State, mainly from the vicinity of Provo, Ogden, and Salt Lake City, as well as the Virgin River in the southwestern portion of Utah, and one record from southeastern Utah (Hayward et al. 1976, p. 107). Reports of nesting birds since 1990 are intermittent, and include locations on the Virgin, Duchesne, Green, San Juan and Colorado Rivers. Additionally, there are reports from at least five other areas where breeding has been suspected (Owens 1998, pp. 3–6). Avian surveys of riparian habitats within the historical range (the Salt Lake Valley) recorded 3 western yellow-billed cuckoos in 7,000 survey hours (Owens 1998, pp. 3–6). No statewide systematic surveys for western yellow-billed cuckoos have been conducted. Survey results and the available literature indicate an extremely small breeding population of western yellow-billed cuckoos in Utah. Therefore, we conclude that the number of breeding pairs in the State is likely fewer than 10 and not likely more than 20 pairs.

Nevada The historical status of the western yellow-billed cuckoo in Nevada is poorly documented, although there is evidence the species nested in western Nevada along the lower Truckee and Carson Rivers and in southern Nevada along the Colorado and Virgin Rivers (Linsdale 1951, p. 235; Neel 1999, pp. 118–120).

Surveys using call-playback techniques were completed along the Truckee, Carson, and Walker Rivers in the early 1970s. In surveys of the six remaining areas of habitat able to support western yellow-billed cuckoos, as described by Gaines (1974, p. 206), no birds were heard or seen (Oakleaf 1974, pp. 18–19). Early documentation of western yellow-billed cuckoos nesting in Nevada included a pair at Beaver Dam Wash, Lincoln County, in 1979 (Neel 1999, p. 119). The only set of persistent sightings along the Carson River occurred on portions near Lahontan Reservoir (Neel 1999, pp. 118–120), where sightings of single birds year after year suggested long-term occupancy from 1986 to 1997 (Tomlinson 2010, p. 1). At least one western yellow- billed cuckoo was detected during surveys at the Lahontan Reservoir delta in 2012 indicating continued residency at that location (Great Basin Bird Observatory 2013, p. 48). Single sightings were also detected by Nevada Division of Wildlife (NDOW) at the Lahontan Reservoir delta in 2016 and 2018, as well as a single sighting at the Pitchfork Ranch along the East Walker River in 2017 (NDOW 2018, p. 8). Between 1990 and 1999, Neel (1999, p. 119) reported only sporadic sightings of single birds throughout the State.

Beginning in 2000, annual survey efforts became more consistent in the southern portion of the State. The NDOW (2001, pp. 1–8) conducted surveys in 2000 in southern Nevada and documented 19 western yellow-billed cuckoos, comprising 4 pairs and 11 unpaired birds with no

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nests found. NDOW surveys in 2000 and 2001 detected more birds (19 and 28, respectively) than in subsequent years, with a general decline in detections from 2002 to 2009, although the survey area was smaller because of reduced access to private lands (Tomlinson 2010, p. 1). Surveys conducted at the Warm Springs Natural Area on the Muddy River documented a nesting record for the species in 2000, but also indicated a general decline in bird numbers from 2002 to 2009 (Tomlinson 2010, p. 1). Surveys have continued at Warm Springs with one detection in years 2015 (SWCA 2015 pg. 7), 2016 (SWCA 2016, p. 1), 2017 (SWCA 2017, p. 1), and 2018 (Van Dooremolen 2018, p. 1). Surveys conducted by the San Bernardino County Museum at sites along the Virgin and Muddy Rivers between 2000 and 2008 detected western yellow-billed cuckoos in all but one year, with the number of individuals detected ranging from a low of 3 to a high of 12 (Braden et al. 2009, pp. 1–58). These surveys were resumed by the Southern Sierra Research Station in 2009 and detected one bird at each of two locations: Pahranagat Valley and the Key Pittman Wildlife Management Area (Tomlinson 2010, p. 2). Pahranagat Valley and Key Pittman WMA continue to be surveyed. Key Pittman had one detection in 2016 (SWCA 2016, p. 1), and Pahranagat NWR had detections in 2015 (SWCA 2015, p. 5), 2016 (SWCA 2016, p. 1), and 2017 (SWCA 2017, p. 1). Incidental western yellow-billed cuckoo detections were also made during other bird surveys in the Pahranagat Valley in 2008, 2010, and 2012 (SWCA 2013, Table C–1). The Meadow Valley Wash is surveyed annually and had western yellow-billed cuckoo detections in 2016 (NDOW 2019, p. 4) and 2017 (NDOW 2019, p. 4).

In 2006, surveys were conducted for the species at four Nevada sites within the Lower Colorado River Multi-Species Conservation Plan Boundary area (Johnson et al. 2007, pp. 1–220), resulting in detection of eight western yellow-billed cuckoos (Johnson et al. 2007, pp. 13–16). Fairly extensive surveys of potential habitat at the Ash Meadows NWR resulted in detection of single western yellow-billed cuckoos in 2008 and 2009 (Tomlinson 2010, p. 2). Additional protocol surveys were conducted in 2009 and 2010 in southern Nevada along the Muddy and Virgin Rivers, resulting in the detections of 3 western yellow-billed cuckoos at Overton Wildlife Management Area along the Muddy River and 1 western yellow-billed cuckoo detection at Mormon Mesa along the Virgin River in 2010 (McNeil et al. 2010, pp. 27–29; McNeil et al. 2011, pp. 140–142). In addition, incidental detections of western yellow-billed cuckoos were made almost annually during other bird surveys along the Virgin and lower Muddy Rivers between 2008 and 2012 with the highest number of 4 western yellow-billed cuckoos occurring in 2010 (SWCA 2013, Table C–1). Annual surveys continue at Overton WMA and along the Virgin River between Mormon Mesa and Mesquite. Overton WMA detected one western yellow-billed cuckoo in 2015 (Bureau of Reclamation (Reclamation) 2019, p. 9), 2016 (SWCA 2016, p. 1), and 2018 (Van Dooremolen 2018, p. 1), and sites along the Virgin River detected two western yellow-billed cuckoos in 2016 (NDOW 2016, p. 3; Klinger 2018, p. 1) and 2018 (SWCA 2018, p. 1; NDOW 2019, p. 4) and one western yellow-billed cuckoo in 2017 (Klinger 2018, p. 1). Surveys have also been conducted within the Las Vegas Wash by Southern Nevada Water Authority with a single detection of western yellow-billed cuckoo in 2016 (SNWA 2016, p. 5)

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and 2017 (SNWA 2017, pp. 4–6). Survey results and the available literature indicate a small breeding population of western yellow-billed cuckoos in Nevada. Therefore, we conclude that fewer than 10 breeding pairs of western yellow-billed cuckoo occur in Nevada.

California In California prior to the 1930s, the species was widely distributed in suitable river bottom habitats, and was locally common (Grinnell and Miller 1944, pp. 186–187; Small 1994, pp. 130– 131). Western yellow-billed cuckoos primarily nested in three general areas of the State: (1) Coastal counties from San Diego County near the Mexico border to Sonoma County in the San Francisco Bay region, (2) the Central Valley from Kern County through Shasta County, and (3) along the lower Colorado River (Dawson 1923, pp. 2–7; Grinnell and Miller 1944, pp. 186–187; Gaines and Laymon 1984, pp. 53–58; Small 1994, 130–131). Western yellow-billed cuckoos also bred locally elsewhere in the State, including in Inyo, San Bernardino, and Siskiyou Counties (Grinnell and Miller 1944, pp. 186–187).

The early ornithological literature for California was summarized and evaluated by Gaines (1974a, p. 204; 1974b, pp. 2–4), Gaines and Laymon (1984, pp. 53–58), and Hughes (1999, p. 4). Collectively, they report 42 locations where the western yellow-billed cuckoo was historically reported or collected in abundance, but is no longer found today. Laymon and Halterman (1987b, p. 24) estimated at the time that the geographical range of the western yellow-billed cuckoo in California was about 30 percent of what it was historically. Hughes (1999, p. 2) provided an estimate of 15,000 breeding pairs in California during the late 19th century. Gaines (1974, p. 208) believed that predevelopment western yellow-billed cuckoo populations in California were even greater than implied by the early literature, due to the species’ inconspicuous behavior and the fact that large tracts of floodplain riparian habitat had already been lost to before the first records and accounts of the species began appearing in published literature. Most modern investigators believe that the initial decline of the western yellow-billed cuckoo population in California occurred following the major era of development that began about the mid-1800s (Gaines and Laymon 1984, p. 73; Laymon and Halterman 1987b, pp. 19–25; Launer et al. 1990, pp. 2–3). The species was listed by the State of California as threatened in 1971, and was reclassified as endangered in 1987.

Western yellow-billed cuckoos historically were recorded from every county in the San Joaquin Valley region except Kings County, and were locally common as a breeding bird at least in San Joaquin, Kern, Fresno, and Stanislaus Counties (Gaines and Laymon 1984, p. 66). The species’ population no longer breeds in the San Joaquin Valley. The last nesting record for this region was in 1974 on Lewis Creek near Lindsey, Tulare County (Laymon and Halterman 1987a, p. 24).

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The first statewide survey for western yellow-billed cuckoos was conducted in 1977; it detected 121 to 163 pairs of western yellow-billed cuckoos during 44 days of survey effort (0.55–0.74 western yellow-billed cuckoo pairs per survey hour)(Gaines and Laymon 1984, p. 77; Halterman et al. 2001, p. 47). The second statewide survey, conducted in 1986 and 1987 with 124 days of survey effort, estimated 32 to 42 breeding pairs in the State, a decline of 66–81 percent from the 1977 survey (0.05–0.07 western yellow-billed cuckoo pairs per survey hour)(Gaines and Laymon 1984, pp. 59–72; Laymon and Halterman 1987a, p. 7). The third statewide survey, in 1999 and 2000, was conducted over 134 days, and estimated 39 to 43 breeding pairs (0.06 western yellow-billed cuckoo pairs per survey hour), a similar population level to 1987, but lower than 1977 (Halterman et al. 2001, p. 47) (Figure 1). The main difference in the most recent statewide survey (1999 to 2000) when compared to earlier surveys (1977 and 1987) was the absence of western yellow-billed cuckoos at isolated sites in the Prado Flood Control Basin in Riverside County, the Mojave River in San Bernardino County, and the Amargosa River in Inyo and San Bernardino Counties, and the Owens Valley in Inyo County where they had previously bred, indicating a contraction of the range to the core areas of occurrence in the State along the Sacramento, Kern, and Colorado Rivers (Dettling et al. 2015, pp. 7–14). In all, the California population of the western western yellow-billed cuckoo today is less than 1 percent of its estimated historical population size. While modern detections at these sites are few, there is some anecdotal evidence since the 2014 listing that a few of these peripheral sites get some (albeit inconsistent) use (Clark et al. 2014, entire). For example, Clark et al. (2014, p. 148) list several locations with multiple western yellow-billed cuckoo detections since 2000, including (in part) the upper Owens Valley, the Amargosa River, the Prado Basin, the San Luis Rey River in San Diego County, and San Felipe Creek in San Diego County. Additionally, two adults were observed feeding a juvenile along the Mojave River in 2017 (Nerhus 2019, in litt.). This observation provides the only evidence of nesting from any of these peripheral sites in recent times, but dedicated survey efforts may yet yield evidence of nesting at some sites in some years.

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Figure 1. Western yellow-billed cuckoo pairs per survey effort on California statewide surveys 1977 – 2000.

Western yellow-billed cuckoos have been considered accidental in coastal northern California; however, from 2000 through 2012, surveys and anecdotal observations along the lower Eel River in Humboldt County detected western yellow-billed cuckoos, and breeding was probable during at least two of those years (McAllister et al. 2010, pp. 1–6). If nesting is confirmed, this would document a new breeding site in the State. Based on statewide survey results, only three areas in the State support more than a few breeding pairs on a regular basis: (1) The Sacramento River (roughly between Colusa and Red Bluff), (2) the South Fork of the Kern River upstream of Lake Isabella, and (3) the lower Colorado River (Laymon and Halterman 1987a, pp. 1–18). Results of surveys and population trends for these sites are summarized below.

Sacramento River—Grinnell and Miller (1944, pp. 186–187) listed the western yellow-billed cuckoo as a common to fairly common breeder in the Sacramento Valley. Gaines and Laymon (1984, pp. 59–60) summarized historical occurrence in the Sacramento Valley, and cited Cooper (1870, pp. 371–373) who found the species quite common in the vicinity of Sacramento in 1865 and Belding (1890, p. 87) who found them common in the vicinity of Marysville in 1878. Gaines (1974, pp. 204–205) conducted the first surveys for western yellow-billed cuckoos on the Sacramento River between Red Bluff and Colusa during 1972, and found 28 individuals at 15 sites. The following year (1973) he repeated this survey, and found 29 western yellow-billed cuckoos at 21 sites (40 survey hours) (Gaines and Laymon 1984, p. 59). During a statewide western yellow-billed cuckoo survey in 1977, researchers found 44 western yellow-billed cuckoos at 29 sites in this same stretch of the Sacramento River, but with greater survey effort (60 survey hours) (Gaines and Laymon 1984, pp. 59–62). From these surveys it was estimated that 29 to 60 pairs of western yellow-billed cuckoos nested along the Sacramento River in 1977.

The Sacramento River was surveyed again in 1987, and a much lower population of 18 to 22 pairs was found despite a more intense survey effort (128 survey hours) (Laymon and Halterman 1987a, p. 6). Halterman (1991, p. 24) continued surveys on the river for 3 additional years with even greater survey effort (255 survey hours each year), and found breeding populations of 35 pairs, 26 pairs, and 23 pairs in 1988, 1989, and 1990, respectively. Surveys in 1999 found 28 to 32 pairs of western yellow-billed cuckoos, and surveys in 2000 located 35 to 40 pairs (Halterman et al. 2001, p. 39). The most recent survey on the Sacramento River, conducted in 2010, located only 16–18 western yellow-billed cuckoos at 48 sites, despite many more hours of surveying effort (1,191 survey hours) (Dettling and Howell 2011, p. 31).

Western yellow-billed cuckoo populations have declined on the Sacramento River in the past 45 years. In the 1970s a western yellow-billed cuckoo was found about once every 1.4 hours of survey effort. During the 1980s a western yellow-billed cuckoo was found half as often with one

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every 2.8 hours of survey effort. From 1990 to 2000 a western yellow-billed cuckoo was found every 2.9 hours of survey effort, but in 2010 it took 66.2 hours of survey effort to locate a western yellow-billed cuckoo (Figure 2). Western yellow-billed cuckoos still occupy this site, but the population has declined by at least 80 percent over the past 35 years, with a major continuing decline in the most recent 10 years. As of 2018, there have been several large habitat improvement projects on the Sacramento River (Silvera, 2018). The extent of habitat on the river has remained stable or increased; however, much of the habitat along the Sacramento River and its tributaries which is available for use by the western yellow-billed cuckoo is currently unused (Dettling et al. 2015, pp. 7–14).

Figure 2. Western yellow-billed cuckoo detection during surveys on the Sacramento River on 10 separate years from 1972 to 2010.

South Fork Kern River—The 3,300-ac (1,335-ha) riparian forest in the South Fork Kern River Valley is one of the largest remaining contiguous tracts of riparian habitat in California. This site has been the most regularly surveyed of any of the western yellow-billed cuckoo breeding locations in California. The species’ occurrence at this site was first documented in 1911 by a specimen collected by Grinnell’s Mount Whitney Expedition (MVZ Birds #19836, Museum of Vertebrate Zoology, University of California (UC) Berkeley). Gaines (Gaines and Laymon 1984, p. 64) rediscovered this population, finding nine individual western yellow-billed cuckoos there during his 1977 statewide survey of the species. From 1985 through 2001 this population was intensively monitored, and the number of pairs and most nests found each year were documented (Laymon and Williams 2001, p 4; Laymon and Williams 2002, p. 5). During this period, the

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population fluctuated from a low of 2 pairs in 1990 to a high of 24 pairs in 1992, with a yearly average of 10.6 pairs.

From 2002 to 2004 and 2008 to 2010, the population was surveyed less intensively and fewer nests found (Halterman 2003, p. 10; Halterman 2004, p. 10; Henneman 2008, pp. 8–10; Henneman 2010, pp. 8–10; Whitfield and Stanek 2011, pp. 8–10). The number of western yellow-billed cuckoo pairs is no longer being estimated, but from reviewing the location of the survey sightings, approximately 8 to 14 pairs (with an average of 10.5 pairs) have nested in the area during this period. From the available survey data and literature, this small breeding population currently appears to still be using the area but numbers have recently declined (Frost 2018, entire). Most of the population is currently nesting on the U.S. Army Corps of Engineers (USACE), U.S. Forest Service (USFS) South Fork Wildlife Area in the western third of the site. The eastern two-thirds of the site is sparsely occupied, and it appears that not all of the potential nesting habitat is currently being used (Henneman 2008, pp. 8–10; Henneman 2010, pp. 8–10; Whitfield and Stanek 2011).

Lower Colorado River—The lower Colorado River on the California-Arizona border supported an estimated 180 western yellow-billed cuckoo pairs during the first California statewide western yellow-billed cuckoo survey in 1976 to 1977 (Gaines and Laymon 1984, p. 72). When the second California statewide survey was conducted in 1986 western yellow-billed cuckoos had decreased by 80–90 percent (Laymon and Halterman 1987a, pp. 34–35). Another study (Rosenberg et al. 1991, p. 203) estimated a decline of 93 percent over this same time period, from an estimated initial 242 pairs in 1976 to 1977. Final results from a Service-funded 1999 statewide survey found only two pairs of western yellow-billed cuckoos on the California side of the Colorado River (Halterman et al. 2001, p. 19), an area where 44 western yellow-billed cuckoos were found in 1977 (Gaines and Laymon 1984, pp. 64–65).

In 2006, surveys were conducted at various sites throughout the Lower Colorado River Multi- Species Conservation Plan Boundary area for the western yellow-billed cuckoo (Johnson et al. 2007, pp. 1–220). Two survey areas were on the California side of the lower Colorado River, the Picacho State Recreation Area and the Imperial NWR (Imperial Paradise area); only one bird was detected, at the Picacho State Recreation Area, Imperial County (Johnson et al. 2007, p. 25). During 2010 and 2011, western yellow-billed cuckoos were found at two locations on the California side of the river which included Picacho State Recreation Area and at the newly created restoration habitat at Palo Verde Ecological Reserve (PVER) in Riverside County The estimated number of territories at these locations are included in the Lower Colorado synopsis for the state of Arizona.

Since the 2014 listing rule, the number of western yellow-billed cuckoos on the lower Colorado River has varied, as has the level of survey effort. Additionally, as studies have continued, it has

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become increasingly clear that western yellow-billed cuckoo nesting behaviors (such as, polyandry, polygyny, and renesting) make estimation of the number of “pairs” difficult, such that recent population numbers do not compare with the pair estimates from previous studies (Reclamation, 2019, p. 43). In general, despite the initial successes at restoration sites, such as the Palo Verde Ecological Reserve, western yellow-billed cuckoo numbers appear to have declined overall (Wilsey et al. 2017, p. 14; Reclamation, 2019, p. 70). This appears to be a product of two factors: (1) prolonged drought, affecting natural habitat areas in particular (Reclamation, 2019, pp. 75–78), and (2) reduced levels of use of restored areas by western yellow-billed cuckoo as the vegetation matures (Reclamation, 2019, p. 76).

Summary of California Status

Western yellow-billed cuckoo numbers in California have declined by more than 99 percent from historical levels and are estimated to be 18 percent of levels observed in the late 1970s. Current nesting populations (areas which have consistently supported numerous nesting pairs) in the State are found at only 3 locations (Sacramento River, Kern River, and the California side of the lower Colorado River). Declines in observed nesting numbers are continuing, especially along the Sacramento and Kern Rivers and at isolated sites that recently supported nesting, but are now not being used. Statewide numbers are most likely lower than 40 to 50 breeding pairs (excluding the lower Colorado River population which is included in Arizona estimates), down from approximately 280 pairs as recently as 1977 and perhaps as many as 15,000 pairs prior to the increased human settlement in the 1850s.

Arizona The western yellow-billed cuckoo was historically widespread and locally common in Arizona (Phillips et al. 1964, p. 45; Groschupf 1987, p. 7). A 1976 study based on existing habitat and known western yellow-billed cuckoo population densities estimated 846 pairs were present on the lower Colorado River and its five major tributaries in Arizona (Groschupf 1987, pp. 20–28). In a statewide survey of selected sites in 1999 that covered 265 mi (426 km) of river and creek bottoms, 172 western yellow-billed cuckoo pairs and 81 single birds were located in Arizona (Corman and Magill 2000, pp. 9–10). While this survey did not cover all potential western yellow-billed cuckoo habitat in Arizona, it indicated that the number of western yellow-billed cuckoos in 1999 was substantially lower than previous estimates for the State. However, Arizona still contains the largest remaining western yellow-billed cuckoo population among the States west of the Rocky Mountains, and the species is considered a Species of Concern by the Arizona Game and Fish Department, a designation that does not provide protection to the species (Corman 1999, p. 1). As habitat has declined, western yellow-billed cuckoo numbers have likely declined, as has been documented for the lower Colorado River (Rosenberg et al. 1991, pp. 202– 205) and described above for California.

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Western yellow-billed cuckoo populations greater than 10 territories are found in the following riparian habitat locations in Arizona: PVER on the lower Colorado River, near Fort Thomas and other sites on the Gila River, San Pedro River, Verde River, Sonoita Creek, Santa Cruz River south of Tucson, Upper Cienega Creek, and Agua Fria River complex. Sites with smaller populations are found at the Roosevelt Lake Complex, Upper Tonto Creek, Pinto Creek, Aravaipa Creek, Granite Creek, Oak Creek, Lower Cienega Creek, Hassayampa River, Babocomari River, Pinal Creek, Bonita Creek, Arivaca Lake complex, Leslie Canyon and San Bernardino NWR, Hooker Hot Springs, Big Sandy River, Santa Maria River, many drainages on Coronado National Forest, some smaller drainages on Tonto National Forest, several drainages within Buenos Aires NWR, Sopori Wash, and many smaller drainages. Nearly all drainages have not been thoroughly surveyed, and it is likely that some additional western yellow-billed cuckoo locations will be discovered. The status of western yellow-billed cuckoos in many areas across the state is unknown, including but not limited to unsurveyed drainages within the desert, foothills, and mountain ranges of southeastern Arizona, White Mountains drainages, Eagle Creek, and along parts of the Gila, Little Colorado River, San Francisco, and Blue Rivers.

Western yellow-billed cuckoo sightings reported by birders between 15 June and 31 August, 1998 to 2018, in more than 1 year in southeastern Arizona mountain ranges include Carr Canyon, Ash Canyon, Garden Canyon, Ramsey Canyon, and Miller Canyon in the Huachuca Mountains; Walker Canyon, Madera Canyon, and Montosa Canyon in the Santa Rita Mountains; Scotia Canyon and Sycamore Canyon in the Atascosa/Pajarito Mountains; French Joe Canyon in the Whetstone Mountains; Harshaw Canyon and Paymaster Spring in the Patagonia Mountains; Kitt Peak on Baboquivari Mountain; and a few locations in the Chiricahua Mountains (Bird05 listserve, 2012). Western yellow-billed cuckoos are breeding in at least some of these locations, with nesting confirmed at Sycamore Canyon (Arizona Game and Fish Department, unpublished data).

The Arizona Breeding Bird Atlas recorded western yellow-billed cuckoos on 50 of 1,834 blocks (2.7 percent), illustrating the species’ rare status. Western yellow-billed cuckoos were confirmed breeding and probably breeding on 29 of these blocks, and possibly on 21 blocks (Corman and Wise-Gervais 2005, pp. 202–203). Multi-year surveys conducted at locations are discussed below. We are in the process of compiling data and anticipate adding information on additional drainages in a future draft.

Territories by Drainage or Mountain Range

Lower Colorado River—As indicated in the California synopsis of the lower Colorado River above, this area supported an estimated 180 western yellow-billed cuckoo pairs in 1976 to 1977 (Gaines and Laymon 1984, p. 72). In 1986, this population had declined by an estimated 80–90 percent (Laymon and Halterman 1987a, pp. 34– 35). In 2006 and 2007, playback surveys

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documented 180 and 163 western yellow-billed cuckoo detections at 55 and 40 sites, respectively throughout the Lower Colorado River Multi-Species Conservation Plan Boundary area, with no estimate of the number of territories (Johnson et al. 2007, pp. 1–220; Johnson et al. 2008 p. 106- 107). The Bill Williams River NWR accounted for the majority of cuckoo detections during both years; 68 percent and 85 percent in 2006 and 2007, respectively (Johnson et al. 2008, pp. 1, 107). At the time, the Bill Williams River NWR represented the largest stand and highest quality of suitable yellow-billed cuckoo habitat on the lower Colorado River. Breeding was confirmed during both years at Bill Williams Rver NWR and at the Grand Canyon National Park/Lake Mead National Recreation Area only in 2006 (Johnson et al. 2008, p. 107).

At Topock Marsh, habitat quality has declined due to the 2016 Topock Fire and damage by the introduced tamarisk leaf beetles but suitable cuckoo habitat still exists. A cuckoo was observed in 2014 and 2018 during protocol surveys and during southwestern willow flycatcher surveys in 2008, 2009 and 2010 (Garcia and Associates 2018, pp. 2-5, 9). All observations were of a single individual. No cuckoos were observed in 2012 and no surveys were conducted in 2013 or 2016 (Garcia and Associates 2018). The Colorado River and Topock Marsh provide standing surface water throughout the breeding season, and the site includes a suitable vegetation composition of contiguous riparian habitat with a dense multi-story canopy composed primarily of native riparian species. The most abundant plant species are coyote willow (Salix exigua), tamarisk (both T. ramosissima and T. aphylla), catclaw acacia (Senegalia greggii), and arrowweed (Pluchea sericea). Coyote willow is the most dominant plant species throughout the survey area.

In addition to the Bill Williams River NWR, other sites in Arizona where Johnson et al. (2008, p. 29) detected western yellow-billed cuckoos in 2006 and 2007 include: the Havasu NWR, Cibola NWR, Imperial NWR, Gila-Colorado River confluence, Limitrophe Division, and Quigley Pond Wildlife Management Area (Johnson et al. 2008, p. 107). Beginning in 2008, the study area and methods again changed, making comparisons with past years more difficult. Sites included the Muddy, Virgin, lower Colorado, Bill Williams, and Gila rivers, (41 to 62 sites including 15 LCR MSCP restoration sites (within 6 conservation areas), 14 non-LCR MSCP restoration sites, 16 Bill Williams River sites, and 32 other natural riparian sites (McNeil et al. 2013, p. ES-1). The number of territories in the study area increased from 47 in 2008 to 80 in 2012 due to the addition of newly available (LCR MSCP restoration) habitat (McNeil et al. 2013, p. ES-2). Planting revegetation sites began in 2006, and within the planted areas, the number of territories increased from 3 in 2008 to 29 in 2012 (Parametrix, Inc. and Southern Sierra Research Station 2019, p. 72, pp. ES-2-3). The increasing trend continued in 2013, with 61 breeding territories (“territories” refers to estimated breeding pairs after formal presence/absence surveys conducted and include more than one cuckoo detection) in LCR MSCP conservation areas, including 51 territories at restoration sites, including 34 at PVER and 8 at Cibola Valley Conservation Area (CVCA) (McNeil et al. 2013, p. 16). Eight territories were also found the Bill Williams River NWR in 2013. Following these initial YBCU monitoring projects (2008–13), surveys continued

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from 2014 to 2018 in a subset of LCR MSCP conservation areas, including the Beal Lake Conservation Area, PVER, CVCA, Cibola NWR Unit #1 Conservation Area (Cibola NWR Unit #1), Laguna Division Conservation Area (LDCA), and Yuma East Wetlands (YEW) (Parametrix, Inc. and Southern Sierra Research Station 2019, p. 72, pp. ES-2-3). Areas surveyed in 2014 and 2015 also included non-LCR MSCP conservation areas (or areas since removed from the project) within the study area, including the Muddy River (Overton Wildlife Management Area), Topock (within the Havasu NWR), Bill Williams River-East and Bill Williams River-West, the ‘Ahakhav Tribal Preserve, the Picacho NWR, the Imperial NWR, and Laguna (Mittry). Following the reduction in scope of this project, surveys continued in LCR MSCP conservation areas from 2016 to 2018, and a portion of the Bill Williams River NWR from 2017 to 2018. The total number of territories were 101, 109, 127, 66, and 72 from 2014 to 2018, respectively. However, Muddy River, Havasu NWR, ‘Ahakhav Tribal Preserve, Reach 5, and Mittry were only surveyed in 2014 and 2015 and LDCA was only surveyed in 2016-2018 (Parametrix, Inc. and Southern Sierra Research Station 2019, pp. 72-73).

Of the sites, PVER continues to support the greatest number of breeding territories on the lower Colorado River annually (78, 80, 104, 49, and 51 territories), followed by Cibola NWR (7, 8, 14, 9, 9) documented annually between 2014 and 2018, respectively; and CVCA (3, 5, 6, 3, and 5) between 2014 and 2018, respectively; (Parametrix, Inc. and Southern Sierra Research Station 2019, pp. 72-73). All other sites surveyed were occupied by 2 or fewer territories annually. Based on the most current surveys, the number of territories along the lower Colorado River, including the Bill Williams River and populated areas on the California side, is likely 75-100 territories. Recent population estimates are well below the breeding population in 1977, even though a larger area was surveyed.

Bill Williams River—Because the Bill Williams River flows into the Lower Colorado River, territory numbers are often included in the lower Colorado River population estimates as they have been above. They are separated out here to provide a more detailed history of the rapid recent decline of this river system. In the mid- 1970s, an estimated 57 pairs of western yellow- billed cuckoos bred in the riparian forest of the Bill Williams River Delta (Gaines and Laymon 1984, p. 71). Following the sustained high water levels of 1983 to 1984 and 1986, which inundated and killed most of the cottonwoods and willows along the Colorado River, western yellow-billed cuckoo numbers also declined on the Bill Williams River Delta where similar habitat mortality occurred (Rosenberg et al. 1991, p. 203). In 1987, 17 pairs of western yellow- billed cuckoos were located at this site and a total of 25 to 30 pairs were estimated to be present, a decline of 47 to 56 percent over 10 years (Laymon and Halterman 1987a, p. 32). Surveys were conducted regularly at this site from 1993 to 2002. The breeding population fluctuated from a low of 6 to 9 pairs in 1999 and 8 pairs in 2002 to a high of 28 to 30 pairs in 1993 and 28 to 39 pairs in 2001 (Halterman 2003, p. 32). Surveys conducted at the Bill Williams River NWR in 2006 and 2007 using playback resulted in 117 and 139 total detections, respectively, with no

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estimate of the number of territories (Johnson et al. 2008, p. 107). The number of territories from playback surveys was 37, 28, 31, 23, and 19 from 2008 to 2012, respectively (McNeil et al. 2013, p.70). The number of territories continued to decline to 10, 11, 0, and 3 in 2014, 2015, 2017, and 2018, respectively (Parametrix, Inc. and Southern Sierra Research Station 2019, p. 72). The western yellow-billed cuckoos population at the Bill Williams River NWR between 2008 and 2018 declined from its peak in 2010 to no cuckoos in 2017, likely in response to the lack of flood flows between two managed dam releases in 2005 and 2018 and associated decline in cottonwood and willow habitat (Parametrix, Inc. and Southern Sierra Research Station 2019, pp. ES-7, 65). The population recovered slightly after the 2018 flood release. Protocol surveys along the Bill Williams River outside of the wildlife refuge conducted in 2018 resulted in 2 potential territories (O’Donnell et al. 2019, p. 15-16). Bill Williams River NWR was once considered the largest, highest quality stand of suitable habitat for the western yellow-billed cuckoo along the lower Colorado River (Johnson et al. 2008, p. 106), but has now declined to likely 10 or fewer territories. Data from this site show an important, but drastically declining, breeding population that has not recovered to 1977 levels.

Alamo Lake - Nine incidental cuckoo detections in 2018 along Alamo Lake include one or two possible pairs in Burro Wash (Pellegrini 2018, in litt.). Of the nine detections, four occurred on or prior to June 15, including one of the pairs. Goodding's willow, cottonwood, mesquite, and tamarisk persists at Alamo Lake. Tamarisk leaf beetles and prolonged tamarisk inundation have caused mortality for a portion of the tamarisk. Cattle and burros are also impacting habitat. Cuckoos were found where native vegetation persisted.

Big Sandy River - An estimated four and three western yellow-billed cuckoo pairs were found during southwestern willow flycatcher surveys in cottonwood, willow, or the adjacent mesquite in habitat in the vicinity of the state Highway 93 bridge in 2005 and 2006, respectively (EcoPlan 2005, p.8; 2006, p. 7). In a separate reach and similar to the results of a study conducted in 1998 (Johnson et al. 2010), two potential territories were found during protocol surveys in 2016 in the Middle Big Sandy River, north of Alamo Lake and south state Highway 93 (O’Donnell et al. 2016, pp. 1,6, 21). Only a small part of this river has been surveyed, but within the reaches surveyed in recent past years, six or fewer territories were found.

Agua Fria River - In 2018, surveyors identified seven occupied patches (territories) within the nine surveyed transects. This is a decrease from the six patches identified in 2017 and from the eleven identified in 2016 (Prager and Wise 2018, p. 13). Upper Agua Fria supported three occupied patches, just as it did in 2016. Cuckoos were detected within this patch during three of the four mandatory survey periods indicating probable breeding. Habitat and water availability continue to improve with the exclusion of off-road vehicles since 2010, winter only grazing, and revegetation efforts. A young cottonwood and willow forest is growing in this reach (Prager and Wise 2018, p. 14). Horseshoe Ford North supported a single occupied territory in 2018 with

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possible breeding, which is consistent with 2016 but is an increase from 2017 (Prager and Wise 2018, p. 15). Reliable surface water is present in this reach, even in dry years. Sycamore Creek, Cross Y, and Horseshoe Bar South did not support any occupied patches in 2018, which is consistent with previous years’ survey efforts (Prager and Wise 2018, p. 15). In some areas, off- road vehicles and grazing continue to impact habitat.

Verde River—Surveys conducted in 2004 and 2005 at 37 sites within the Verde River watershed were done at historical sites (16) at locations where western yellow-billed cuckoos were previously detected in 1998 to 1999 and at random sites (21) with riparian forest that appeared to be suitable nesting habitat (Holmes et al. 2008, pp. 6–7). In the 2 years, 59 percent of sites had detections: 75 percent of historical sites and 48 percent of random sites (Holmes et al. 2008, p. v). Holmes et al. (2008, p. 20) confirmed nesting at five sites and found evidence of probable breeding at nine additional sites. The maximum number of detections during any one survey period was 23 in 2004 and 31 in 2005.

Southeastern Arizona Drainages. The North American Monsoon (monsoon), a large scale weather pattern which causes high humidity and a series of summer thunderstorms during the summer in northwestern Mexico and the southwestern United States (Erfani and Mitchell 2014, pp. 13,096-97; National Weather Service 2019 p. 4), creates favorable conditions for western yellow-billed cuckoos in southeastern Arizona. The resulting growth in herbaceous vegetation and production of large insects coincides with peak western yellow-billed cuckoo breeding in perennial, intermittent, and ephemeral drainages in riparian, Madrean evergreen woodland, mesquite woodland (mesquite thorn forest) habitats. These include many ephemeral and intermittent drainages in the bajadas, foothills, and mountains including those in the Catalina Mountains, Rincon Mountains, Santa Rita Mountains, Patagonia Mountains, Huachuca Mountains, Pajarito/Atascosa Mountains, Canelo Hills, Guadalupe Canyon, Buenos Aires NWR, and Leslie Canyon and San Bernardino NWR. Western yellow-billed cuckoos in these more arid drainages are more sparsely distributed than in major drainages like the Santa Cruz River, San Pedro River, Cienega Creek, and Sonoita Creek (Corman and Magill 2000, pp. 37–48; WestLand Resources, Inc. 2013a, pp. 3–5; Westland Resources, Inc. 2013b, pp. 1–9; Griffin 2015, pp. 17– 25; MacFarland and Horst 2015, pp. i–iii, 2, 5–7; Tucson Audubon 2015, p. 44; WestLand Resources, Inc. 2015a, pp. 3–4; WestLand Resources, Inc. 2015b, pp. 3–4; WestLand Resources, Inc. 2015c, entire; Cornell Lab of Ornithology 2019 (eBird data), MacFarland and Horst 2018, in litt.; Corson 2019, pp. 6, 10-13, 18; Sferra et al. 2019, pp. 3, 9-11).

Upper San Pedro River—This site has had the largest western yellow-billed cuckoo population in Arizona. Western yellow-billed cuckoos were surveyed on 42 mi (67 km) of riparian habitat on the upper San Pedro River for 7 years from 2001 to 2007 (Halterman 2002, pp. 10, 22; Halterman 2003, pp. 9, 23; Halterman 2004, pp. 9, 33–34; Halterman 2005, pp. 8, 22–23; Halterman 2006, pp. 26–27; Halterman 2007, pp. 5, 11; Halterman 2009, p.23). The number of

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surveys varied from year-to-year with one to five surveys per year and with different methods used to determine population size. In 2001, researchers estimated a total of 40 to 52 pairs, and 29 to 50 pairs the next year. A total of 26 or more pairs was estimated in 2003, but the number of pairs was not estimated after that year. Year-to-year comparisons were made by summing the maximum number of western yellow-billed cuckoos in each transect for each year, which yields a minimum population of individual western yellow-billed cuckoos over the breeding season.

Survey effort level varied between years and locations, but the number of individuals by year are as follows 71 in 2001, 114 in 2002, 128 in 2003, 101 in 2004, 76 in 2005, 47 in 2006, 83 in 2007. Other western yellow-billed cuckoo populations have shown annual fluctuation in detections (Halterman 2007, p. 23). Intensive western yellow-billed cuckoo surveys have not been conducted at this site since 2007. In addition to numbers reported above, during 2001 and 2002, researchers detected 36 and 81 incidental western yellow-billed cuckoos, respectively, along the San Pedro River during southwestern willow flycatcher surveys (EEC 2002, pp. 6, 12, 13). However, these are detections only, with no determination of number of territories. A repeat of these southwestern willow flycatcher surveys in 2009 detected 26 incidental western yellow- billed cuckoos (The Vernadero Group 2009, pp. 9, 19).

Sonoita Creek - A 4-mi (6-km) segment of Sonoita Creek was surveyed in 7 years between 1976 and 1986 (Groschupf 1987, p. 14). Western yellow-billed cuckoo pairs were not estimated, but lows of 5 and 6 individuals were found in 1976 and 1986, respectively, and highs of 24 to 28 individuals were found between 1977 and 1979. The site was surveyed again in 1998 and 1999, with 11 to 12 pairs and 8 to 9 single western yellow-billed cuckoos located (Corman and Magill 2000, pp. 39–40). In 2005, 17 individuals were found while conducting bird surveys for Important Bird Area designation (Arizona Audubon 2012, http://iba.audubon.org/ iba). This population, while fluctuating, does not appear to have decreased in size from 1976 to 2005. No recent western yellow-billed cuckoo surveys have been conducted at this site.

San Bernardino and Leslie Canyon NWR - Seven territories were detected during protocol surveys in 2016 along Black Draw and Hay Hollow and one territory was found in Leslie Canyon (Radke 2016, p. 41). In 2018, one to four cuckoos were detected on each of four surveys, with likely at least one breeding territory (Cajero 2018, entire).

Buenos Aires NWR - Western yellow-billed cuckoo surveys have been conducted in only a portion of suitable habitat on the refuge. Six of 10 drainages where protocol surveys were conducted yielded 13 occupied territories during the breeding season: Lower Brown Canyon, Brown Wash, Cuadro Wash, Canoa Wash, Las Guijas Wash, and Santa Margarita Wash in drainage bisecting desert and semi-desert scrub and grassland and Madrean evergreen woodland (Corson 2019, p. 10-13). Nesting was confirmed in Lower Brown Canyon and Canoa Wash and a juvenile was documented in Cuadro Wash. Mesquite, acacia, hackberry, and oak were the

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dominant tree species in occupied drainages (Corson 2019, pp. 6, 18). In 2015, an intensive study documented 9 territories and nesting in Arivaca Cienega, Arivaca Creek–Honnas Pond, and Arivaca Creek (Griffin 2015, pp. 19, 22).

Southeastern Arizona Foothills and Mountain Drainages - Protocol surveys have been conducted in short lengths of different foothill and mountain drainages in Southeastern Arizona in one or more years since 2012, primarily in Coronado National Forest. These arid drainages are more sparsely occupied than perennial drainages with cottonwood and willow habitat, and usually five or fewer territories are found in each drainage. The first western yellow-billed cuckoo oak tree nest was found in August 2014 in Montosa Canyon in the Santa Rita Mountains (Tucson Audubon 2014, in litt.). In a 2015 protocol survey, at least 26 territories were detected in the Atascosa Mountains (Pena Blanca Lake, Arivaca Lake, Rock Corral Canyon, Sycamore Canyon), Canelo Hills (Collins, Korn, Lyle canyons); Santa Rita Mountains (Montosa and Box canyons, Proctor Road), Whetstone Mountains (Guindani Canyon), Santa Catalina Mountains (Peppersauce Canyon), Patagonia Mountains (Washington Camp, Harshaw Creek, Sycamore Canyon), Huachuca Mountains (Merritt Canyon) (MacFarland and Horst 2015, entire; Sferra 2015, in litt.). Another 18 western yellow-billed cuckoos were present on only one survey visit in locations at Arivaca Lake, Pena Blanca Canyon, Pena Blanca Lake, Rock Corral Canyon, Sycamore Canyon, Hunter Canyon, Miller Canyon, Finley and Adams Canyon, Flying R Ranch and Paymaster Creek, Sycamore Canyon, Box Canyon, Florida Canyon, and French Joe Canyon; additional effort would have been required to determine how long these individuals remained in the area. In this same study, no birds were found where surveys were conducted in Sabino Canyon in the Santa Catalina Mountains, Sunnyside and Scotia Canyons, Ramsey Canyon, and Carr Canyon in the Huachuca Mountains; Endless Chain Canyon in the Patagonia Mountains and the Cave Creek Complex in the Chiricahua Mountains. Several additional drainages in the Santa Catalina Mountains were surveyed only on one morning with no detections, but additional surveys would be necessary to confirm whether or not these drainages remained without cuckoos all summer. In a 2018 study to document western yellow-billed cuckoo occupancy and breeding, approximately 44 territories were documented in ephemeral drainages in the Santa Catalina Mountains (Peppersauce Canyon, Tanque Verde Wash, Catalina State Park); Santa Rita Mountains (Florida, Box, Chino, Mid Temporal, Upper Temporal, and Smith Canyons); Canelo Hills (Cherry, Lyle, Post canyons), Patagonia Mountains (Corral Canyon, Harshaw Creek, Willow Springs Canyon, Goldbaum Canyon; Cherry Creek); and Atascosa Mountains (California Gulch, Sycamore Canyon, Rock Corral Canyon, Fresnal Wash) (Sferra et al. 2019, pp. 3-7). In this same study, 18 nests, fledglings, or juveniles were confirmed in Peppersauce Canyon, Tanque Verde Wash, Florida Canyon, Box Canyon, Chino Canyon, Mid Temporal, Upper Temporal, Smith Canyons, Cherry, Harshaw Creek, Willow Springs Canyon, Goldbaum Canyon; Cherry Creek, Sycamore Canyon, Rock Corral Canyon, and Fresnal Wash. Two nests each were found in Box Canyon and Florida Canyon in the Santa Rita Mountains in 2018. Cuckoos were nesting in Sycamore Canyon in the Atascosa Mountains into September.

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In 2017, cuckoos were present in three ephemeral drainages in the Atascosa Mountains that drain into the Santa Cruz River near the U.S./Mexico border late into the season (MacFarland 2017, in litt.). Breeding was confirmed in one canyon and likely in another. No cuckoos were detected in Bellatosa Canyon until July 19, with one detection. Four and five cuckoos were detected on the next two surveys, respectively, with up to 4 territories present for the field season. Breeding was confirmed with a pair carrying nesting material on August 18, 2017. No cuckoos were detected in Mariposa Canyon until August 18, when 6 cuckoos were present, including vocal exchanges between two birds. Three cuckoos were detected at this site during late cuckoo survey on September 15. Unknown is whether these late season birds were migrants. Cuckoos in Pesquiera Canyon were present on July 19 (3 birds), August 2 (2 birds), and August 18 (1 bird). Birds were likely breeding, with soft alarm calls on July 19 and aggressive behavior in response to playback on August 18.

Canelo Hills - This unique area is composed of a mix of habitats including semi-desert uplands, oak savannahs, oak woodlands intersected by small ephemeral creeks with riparian and xeroriparian habitat. Cuckoos occupy the Audubon Research Ranch, Post Canyon, and Cherry Creek annually in small numbers. Breeding has not been confirmed, but is likely, given the continued presence of cuckoos during the breeding season. The timing and probability of breeding is dependent on the amount of monsoonal rain during the summer. Encroachment of exotic grasses and shrubs, past fire suppression activities, and human use have contributed to high wildfire risk (Tucson Audubon 2019; http://aziba.org/?page_id=199) Surrounding development and groundwater pumping are likely to increase, possibly impacting cuckoo habitat.

The available literature and surveys suggest that western yellow-billed cuckoo populations in Arizona have declined in many perennial riparian areas from historical levels as well as over the past 35 years have declined, with recent declines at some of the largest populations (for example, Bill Williams River). However, data gathered from surveys and incidental detections conducted since listing indicate western yellow-billed cuckoos are breeding in a broader range of habitats and geographic areas in Arizona than previously known, including intermittent and ephemeral drainages with a mix of xeroriparian and nonriparian trees. Western yellow-billed cuckoos are more sparsely distributed in these more arid drainages than in perennial riparian drainages. Fewer than 10 territories are present within most drainages, but with so many drainages containing cuckoos they make up a large amount of occupied habitat across the landscape. At present, the estimated number of known territories is a minimum of 350 territories (breeding pairs or single birds present during the breeding season) excluding the lower Colorado River and 450 territories including the lower Colorado River. This minimum number of territories is based on data from the Arizona Natural Heritage Database and some additional site-specific studies. We are in the process of compiling data and updating the state estimate, and will provide these

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revised numbers in a future draft. Despite western yellow-billed cuckoo declines in perennial riparian habitat, the population in Arizona is the largest in the United States.

Western New Mexico Western yellow-billed cuckoos were historically common in riparian areas along the Rio Grande, as well as uncommon to common locally along portions of the Gila, San Francisco, and San Juan Rivers (Bailey 1928, pp. 307– 309; Hubbard 1978, p. 32). A habitat analysis and wildlife survey of the middle Rio Grande Valley from Espanola to La Joya estimated that 315 pairs of western yellow-billed cuckoos bred along this river segment (Howe 1986, p. 10).

Surveys have been conducted by Reclamation from 2006 through 2018 along the middle Rio Grande, most recently occurring from Los Lunas downstream to Elephant Butte Reservoir (Dillon et al. 2018 pp. 4-11). The area covered by the surveys increased from 36 mi (58 km) in 2006 to roughly 129 mi (207 km) in 2018. From 2014 to 2017, the area covered by surveys was consistent and throughout that period, estimated pairs ranged from a low of 91 to a high of 116 (Dillon et al. 2018 p. 35). These surveys have documented a sizable population, but many fewer than the 315 pairs estimated for this region in 1984 (Howe 1986, p. 10). The lower Rio Grande has also been recently surveyed in the area from Caballo Reservoir to El Paso, Texas (White et al. 2018 p. i). From 2014 to 2017, estimated territories have ranged from 10 to 28 pairs (White et al. 2018 p. 25).

Systematic surveys have recently also been carried out on the Gila, San Francisco, San Juan, and Mimbres Rivers as well as on Las Animas Creek. The extent of habitat in these areas is limited, and much is discontinuous and fragmented. Based on available habitat and the survey efforts in place, an estimated 70 western yellow-billed cuckoo pairs could breed on the Gila River, while an estimated 10 pairs may breed on the San Juan and San Francisco Rivers, in total (Shook et al. 2017 pp. 7-10). Surveys were completed in 2016 at Las Animas Creek where an estimated 8 pairs were observed. Surveys and incidental detections along the Mimbres River resulted in an estimated 2 pairs. An estimated 190 to 235 western yellow-billed cuckoo pairs currently breed in western New Mexico.

Western Texas The western yellow-billed cuckoo historically was considered to be fairly common in riparian habitat at elevations of 3,000– 7,500 ft (900–2,200 m) in El Paso, Hudspeth, Culberson, and Presidio Counties (Oberholser and Kincaid 1974, pp. 434–435; Rappole and Blacklock 1994, pp. 125–126). Recent information reports that western yellow-billed cuckoos have declined in El Paso County (Peterson and Zimmer, 1998, p. 66). Population reports in the Trans-Pecos area of western Texas near Big Bend National Park show scattered populations of western yellow-billed

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cuckoos (Wauer 1971, pp. 18, 27). These populations tend to be associated with areas of springs and developed wells or earthen ponds that support cottonwoods and willows.

Yellow-billed cuckoo population trends from 1966 to 1998 for the entire State of Texas, eastern and western, show a decline (USGS Biological Resources Division 1999, p. 1). The Texas Parks and Wildlife Department (TPWD) currently does not separate the eastern and western populations of the yellow-billed cuckoo, and identifies the species as globally abundant and State secure since the State ranking was last revised in 1994. However, subsequent publications by the TPWD indicate the species is becoming increasingly rare and declining (Shackelford and Lockwood 2000, p. 1). During 4 years, between 1988 and 1998, a 116-mi (189- km) segment of the Rio Grande (16 mi (26 km) in New Mexico and 99 mi (159 km) in Texas) was surveyed for western yellow-billed cuckoos. The 1988 and 1992 survey results were similar, with western yellow-billed cuckoos responding at 20 of 67 sites and 25 of 109 sites, respectively. The population then dramatically declined, with only 4 western yellow-billed cuckoos at 113 sites in 1995 and 7 western yellow-billed cuckoos at 134 sites in 1998 (Sproul 2000, p. 3). The author concluded that the western yellow-billed cuckoo is a rare, highly vulnerable, and declining species in the Rio Grande Valley of southern New Mexico and extreme west Texas (Sproul 2000, p. 5). Sproul attributed the decline to habitat loss and degradation as well as other unknown factors in the species’ migratory and wintering grounds (Sproul 2000, pp. 3–4). The current population of the western yellow-billed cuckoo in western Texas is likely fewer than 10 pairs.

Northwestern Mexico The western yellow-billed cuckoo breeds locally in northwestern Mexico, and is a widespread transient during migration (Howell and Webb 1995, pp. 346–347). In northwestern Mexico, it has been recorded as a summer resident (presumably breeding), including the extreme northern and southern portions of the Baja California Peninsula, northwest Mexico from Sonora and Chihuahua south to western Durango and Sinaloa (Howell and Webb 1995, pp. 346–347), and irregularly and locally south to western Nayarit and western Zacatecas (World Bird Info 2012).

Baja California Peninsula— Historically, the western yellow-billed cuckoo was a rare and local migrant and summer resident in Baja California and Baja California Sur (Grinnell 1928, p. 119). Miller (1950, p. 83) observed a migrating flock of western yellow-billed cuckoos in the Cape region of Baja California Sur in late May or early June 1896. Lamb (1927, p. 157), during 2 years living in the Cape region, saw western yellow-billed cuckoos on only two occasions, once in late June and again in early September. A recent status review of birds on the Baja California Peninsula listed the species as a probable breeder only along the Colorado River and in the Cape region (Howell 2001, p. 17; Howell et al. 2001, p. 182). The population along the Colorado River was formerly numerous, but now very few western yellow-billed cuckoos can be found (Patten et al. 2001, p. 46). Bird surveys conducted along the Colorado River, Mexico, from May

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2002 to July 2003 concluded that the presence and density of breeding western yellow-billed cuckoos is largely dependent on the state of riparian habitat and presence of water (Hinojosa- Huerta et al. 2008, pp. 75–92). Suitable habitat disappeared from the Rı´o Colorado floodplain in the latter part of the 20th century due to dewatering of this portion of the river. Pulse floods in the 1990s and 2000s promoting cottonwood and willow habitat regeneration resulted in western yellow-billed cuckoos returning to breed once riparian nesting habitat developed. Western yellow-billed cuckoo persistence will depend on dedicated instream flows and pulse floods, maintenance of vegetative cover and structural diversity, and an increase in older riparian stands (Hinojosa-Huerta et al. 2008, pp. 75–92). The population levels of western yellow-billed cuckoos in the Cape Region of Baja California Sur are not known, but they are uncommon and nesting has not been confirmed, although they likely occur annually (Erickson et al. 2013, p. 591).

Sonora—Western yellow-billed cuckoos are a common summer resident in Sonora, and were observed with higher frequency than in adjacent Arizona (Russell and Monson 1998, p. 131). In the vicinity of Alamos in southern Sonora, Short (1974, p. 24) found the species a common to abundant breeder during the rainy season in late July and early August. During general bird surveys in northern Sonora from 2000 to 2007, western yellow-billed cuckoos were detected in 11 of 16 watersheds (Flesch 2008, pp. 35–36). On the Sonoyta River in northwestern Sonora, the species was not found on the lower stretches and was rare upstream on the Vamori section. On Rio de la Concepcion, western yellow-billed cuckoos were not found on the lower river section or the upper or lower Plomo sections. They were rare on the upper and lower Sasabe sections and uncommon on the Altar, Busani, Coyotillo Magdalena, and CocosperaBambuto sections. They were not found on the Santa Cruz River and were uncommon on the San Pedro River. They were also uncommon on the San Miguel and Bacanuchi-Sonora section of the Rio Sonora. The author defined rare as ‘‘present but rarely detected and often restricted to localized area’’ and defined uncommon as ‘‘present but may not be found in a day or two of field observations’’ (Flesch 2008, pp. 35–36).

Western yellow-billed cuckoos were described as fairly common summer residents, probable breeders, on bird transect surveys in riparian, desert scrub, and foothills thornscrub habitat in east-central Sonora conducted in July (2007, 2008, 2012) and September 2007 and 2009 between 1,542–3,773 ft (470–1150 m) in the 45,000-ac (18,211-ha) Northern Jaguar Reserve in the foothills of the Sierra Madre near the town of Sahuaripa in east-central Sonora (Flesch 2009, pp. 5, 9, 12, 16, 21; Flesch et al. 2015, p. 289-290). The reserve, bordered by the Rı´os Aros and Bavispe, is composed of oak forests mixed with native fan palms, dense thornscrub that transitions into subtropical vegetation, mesquite bosque, and perennial streams lined with sycamores. Most of the annual rainfall in this area occurs during the North American Monsoon and the trees and shrubs in the drought deciduous thornscrub are in leaf during and soon after the summer monsoon (Flesch 2015, p. 289).

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Western yellow-billed cuckoo surveys, using playback, were conducted throughout Sonora at 57 sites in one or more years between 2015 and 2018 (Macías-Duarte 2019, entire). Most sites were surveyed twice, once in July and once in August in a given year. Birds were present in at least the July or August survey at 10 of 11 sites in ephemeral desert arroyos, all surveys at 35 sites in riparian habitat, and all surveys at 11 sites in upland habitat. Some riparian habitat sites were not perennial. To provide an indication of bird density, the number of individuals per kilometer was calculated for 2016. Individuals per kilometer in riparian areas, desert arroyos, and uplands averaged 2.94, 2.18, and 1.76 individuals per kilometer, respectively (Sánchez-Murrieta 2017, p. 34). The only sites where birds were not present on one or both surveys were three desert arroyo sites (Macías-Duarte 2019, entire). Numbers of birds were greater during surveys at the beginning of the field season and the end of the field season, which is likely a function of the presence of both migrants and breeding birds at these times (Sánchez-Murrieta 2017, p. 47). Western yellow-billed cuckoo detections were greatest at two sites, both in canyons. The riparian site "Mazocahui" in the Sonora River contains tall cottonwood, mesquite, and willows, with flow throughout the season (Sánchez-Murrieta 2017, p.49). The other canyon site, "San Cristóbal," in an upland habitat bisected by an ephemeral drainage near the town of Arizpe is composed of dense, tall mesquite and other thorn species with regular surface flow during the rainy season. This site is approximately 8 km from the Sonora River and individuals may move between the two sites. Although desert arroyos and upland support fewer individuals than riparian habitats, they may support a greater proportion of the population than riparian habitats in Sonora due to their greater surface area (Sánchez-Murrieta 2017, p. 56).

Breeding western yellow-billed cuckoos were documented from July through September along approximately 60 km (37 mi) of the Santa Cruz River in northern Sonora during riparian bird point count surveys in 2001 and 2003. They were fairly common at sites ranging from typical cottonwood-dominated riparian habitat (with or without understory) to mesquite-oakgrass habitat. The riparian habitat in this region is moderately impacted from water use, vegetation loss, presence of cattle, and land clearing for agriculture (Sonoran Institute 2008; pp. 2, 25, 55).

Western yellow-billed cuckoo call playback surveys conducted from 21 June through 26 September 2003 documented 142 western yellow-billed cuckoos at 10 sites ranging from 1,148 ft to 3,937 ft (350 to 1,200 m). Western yellow-billed cuckoos were found in riparian habitat at Agua Calienta on the Rio Bambuto north of Imuris; Rio Tubutama near Tubutama and La Reforma; Rio Cuchujaqui northwest of Alamos; Rio Sonora at Aconchi and Baviacora, northeast of Hermosillo on the Cananea-Ures stretch of State Highway 116; El Gavilan on Rio Sonora east of Ures; Upper Rio San Pedro near San Pedro Palominas, and near the Ejido José María Morelos in Cananea (IMADES 2003, pp. 4, 14, 20).

Western yellow-billed cuckoo call playback surveys conducted from July through September 2005 documented western yellow-billed cuckoos in northeastern Sonora along the Rios Sonora,

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Bacanuchi, Cajon Bonito, Bavispe, Moctezuma, and Sahuaripa. Habitat consisted of cottonwood, willow, and mesquite (CEDES 2005, pp. 5, 10, 11). Extensive grazing, agriculture, mining and related water withdrawals have reduced the riparian quality on these rivers.

Marshall (1957, p. 74), in his pine-oak woodland bird study in southern Arizona and adjacent Mexico, found the western yellow-billed cuckoo as a migrant or wanderer in riparian timber only once in Sonora in the Ajos Mountains on July 17, 1952. During wildlife surveys by boat and foot in July and August 2005, of the 115-mi (185-km) stretch of the Rios Aros and Yaqui and tributaries from Nátora (2,275 ft (700 m)) to El Rio (1,138 ft (350 m)) in east-central Sonora, western yellow-billed cuckoos were described as common in riparian groves and thorn scrub woodland. They were detected on both side drainages and main river channels (O’Brien et al. 2006, pp. 4, 8, 24, 37, 46, 51).

In a study focusing on cactus ferruginous pygmy-owls during late spring and summer from 2001 through 2010, Flesch (2012 in litt.) found western yellow-billed cuckoos at 95 sites from June to September at elevations from 328 to 6,902 ft (100 to 2,104 m). The number of birds at each site ranged from 1 to 15 individuals. Flesch also confirmed breeding at four sites in thorn scrub habitats and at one site in upland Sonoran Desert habitat. These records indicate a broader use of habitat by western yellow-billed cuckoos in Sonora. Western yellow-billed cuckoos are more common as breeders in southern Sonora where they nest in thorn forest than in the more arid northern Sonora. There is some evidence that western yellow-billed cuckoos may be nesting farther north and then re-nest in southern Sonora and northern Sinaloa during the rainy season in late July and August (Rohwer et al. 2009, pp. 19050–19055), but additional data are needed to confirm where and how commonly this occurs. Western yellow-billed cuckoos appear to breed at higher density, especially in southern Sonora, but the breeding population for the State of Sonora is probably similar to the State of Arizona with 150 to 250 pairs because Sonora is half the size of Arizona. However, some of the western yellow-billed cuckoos that breed in southern Sonora late in the nesting season may have been counted on breeding grounds farther north earlier the same year.

Chihuahua—Most of the State of Chihuahua is desert with very little rainfall and few waterways with significant riparian habitat. The Rio Conchos is the primary river system that drains the southern half of the State. This river is highly degraded, with a high density of nonnative tamarisk and little regeneration of willows and cottonwoods due to extremely heavy grazing. This problem has been worsened by a prolonged drought from the late 1990s to the present. Only one sighting of a western yellow-billed cuckoo is listed on the e-Bird online database for the State of Chihuahua, found on July 1, 2003, along Highway 16 between the city of Chihuahua and the town of Lopez Mateos (Cornell Lab of Ornithology 2012). The breeding population for the State of Chihuahua is likely very low, probably in the low double digits and possibly in the single digits.

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Sinaloa—How far south western yellow-billed cuckoos breed in Sinaloa is uncertain. The only two observations of the species (Cornell Lab of Ornithology 2012) are from extreme northern Sinaloa along the Rio Fuerte. Because a thorough survey has not been conducted, the western yellow-billed cuckoo population in the State is likely higher than these records imply. However, much of the thorn forest and riparian habitat has been converted to industrial agriculture over the past 30 years (Rohwer 2010, p. E16). The breeding population of western yellow-billed cuckoos in Sinaloa is unlikely to exceed that of Sonora (150 to 250 breeding pairs), and it may be less.

Western Durango—Three observations of the western yellow-billed cuckoo (Cornell Lab of Ornithology 2012) have been made for the State of Durango west of the Continental Divide. The population for this region is likely very low, possibly in the low double or single digits.

Population summary in Mexico—The available literature indicates that knowledge about the status of the breeding population of the western yellow-billed cuckoo in Mexico is less certain than in the United States. No systematic State-level surveys for the species have been carried out in any of the Mexican States. General bird surveys in Sonora have found western yellow-billed cuckoos in similar habitats and abundances as in Arizona, as well as in thorn forest and dry deciduous forest, which do not occur north of Mexico. The riparian habitat in Mexico appears to be more fragmented and heavily grazed than it is north of the international border, and the thorn forest habitat that the species is using in southern Sonora and Sinaloa is being converted to industrial agriculture at a high rate. We are in the process of updating western yellow-billed cuckoo numbers using recent survey and incidental detection data and revised numbers will be provided in a future draft. Therefore, we conclude that the western yellow-billed cuckoo in Mexico has a breeding population of 330 to 530 pairs that is likely declining.

Population Summary of the Western Yellow-billed Cuckoo—The available surveys and literature support the conclusion that the population of the western yellow-billed cuckoo has declined by several orders of magnitude over the past 100 years, and that this decline is continuing. Recent declines over the past 15 years have shown both a loss of breeding western yellow-billed cuckoos in smaller isolated sites and declines in numbers at core breeding areas. The current breeding population is low, with 350 to 495 pairs north of the Mexican border and another 330 to 530 pairs in Mexico for a total of 680 to 1,025 breeding pairs. The breeding population may actually be lower than these estimates, as some of these pairs may be counted twice since western yellow-billed cuckoos apparently move into southern Sonora and Sinaloa during the rainy season in late July and August after they have previously bred farther north. Therefore, we conclude that the western yellow-billed cuckoo currently has a small and declining population.

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