14 April 2000

Field Supervisor
U.S. Fish and Wildlife Service
Sacramento Fish and Wildlife Office
2800 Cottage Way, Room W- 2605
Sacramento, California 95825

Dr. Robert G. Amundson, Ecophysiologist
Dr. Jonathan L. Atwood, Antioch New England Graduate School
Dr. A. Joy Belsky, Ecologist, Oregon Natural Desert Association
Dr. Charles T. Collins, California State University, Long Beach
Dr. David Dobkin, Executive Director, High Desert Ecological Research Institute
Dr. Alan Harper, U.S. Co-Chair, Pro Esteros
Dr. Sallie Hejl, U.S. Forest Service, Pacific Southwest Research Station
Dr. Janice M. Hughes, Royal Ontario Museum
Dr. Walter D. Koenig, Research Zoologist, University of California, Berkeley
Dr. Stephen A. Laymon, Conservation Biologist
Gale Monson, Arizona Naturalist
Dr. Reed Noss, President, Conservation Science Inc.; President, Society for Conservation Biology
Dr. Robert Ohmart, Arizona State University
Kim O’Keefe, Department of Biological Sciences, Stanford University
Dr. Frank A. Pitelka, Professor Emeritus, University of California, Berkeley
Dr. Peter H. Raven, Missouri Botanical Garden
Dr. Sievert Rohwer, Curator of Birds, Burke Museum, University of Washington
Dr. Shawn Smallwood, Consulting in the Public Interest
Dr. Thomas B. Smith, Center for Tropical Research, Department of Biology, San Francisco State University
Dr. Peter Stacey, Conservation Biologist, University of New Mexico
Dr. John Terborgh, Duke University Center for Tropical Conservation
Dr. Pamela L. Williams, California Institute for Biodiverstiy

RE: Status of the Yellow-Billed Cuckoo

Thank you for the opportunity to submit comments on the status, distribution, and distinctiveness of the Yellow-billed Cuckoo (Coccyzus americanus). The best available scientific information indicates that cuckoo populations are declining at an alarming and increasing rate throughout much of North America. Hughes (1999) warns that "the future of the Yellow-billed Cuckoo is uncertain. Populations are declining precipitously throughout its distribution." Though most dramatic west of the Continental Divide, alarming cuckoo declines have also occurred east of the divide, especially in the northern portions (Hughes 1999, BBS 2000). Endangered Species Act listing of the cuckoo, as a species, subspecies, or distinct population segment is necessary to arrest the current trajectory toward extirpation in large portions of the species’ range.

The following comments address the subspecific status of C. a. occidentalis, the distinctiveness of the western population, and the significance of declining populations to the species as a whole.

Subspecies. The cuckoo population west of the continental divide has been widely recognized as the Western or California subspecies (C. a. occidentalis) since being described by Ridgway (1887). The American Ornithologists Union (AOU) recognized the Western subspecies in the last Checklist of North American Birds to address subspecies (AOU 1957). The Service’s 90-day finding cites the American Ornithologists Union’s 1998 Checklist of North American Birds (AOU 1998) for the conclusion that the AOU now has a "neutral position" on the validity of the subspecies. AOU (1998), however, does not question the taxonomic validity of the Western Yellow-billed Cuckoo. It merely states in the preface that "some" currently recognized subspecies may not stand up to modern taxonomic study and that others may actually be cryptic species (p xii). The AOU’s general assertion should not be cited as if it were specifically directed to the Western Yellow-billed Cuckoo. We also note that the Service has recently relied on the subspecific conclusions of AOU (1957) to justify its listing decisions for the Northern goshawk, Queen Charlotte goshawk, and other species. It is inappropriate, and gives the appearance of arbitrariness, to rely on AOU (1957) when it agrees with the Service’s decision, yet dismiss the same study when it contradicts the Service’s decision.

The two most recent taxonomic treatments are by Banks (1988, 1990) and Franzreb and Laymon (1993). Banks (1988) concluded that there were no significant differences in morphometric measurements between the Eastern and Western subspecies and that the western subspecies should be suppressed. Banks (1990), however, retracted the statistical analysis because of errors. His corrected analysis demonstrated statistically significant differences (p<0.05) in wing and bill size between eastern and western cuckoos. Nonetheless, he continued to maintain that the western subspecies was not valid because the differences were not large enough. Using discriminate function analysis, Franzreb and Laymon (1993) found significant differences in wing (p<0.00001), tail (p<0.00001), and bill lengths (p<0.05), and bill depth (p<0.05). They were able to accurately identify 89.6% of eastern females, 85.8% of western females, 83.8% of eastern males, and 74.6% of western males. Franzreb and Laymon also described behavioral, vocal, and ecological differences between eastern and western birds, but did not conduct a statistical analysis of these traits. They concluded that subspecific recognition, though not absolutely definitive, should be maintained.

Pyle (1997) recognized eastern and western subspecies, presenting a formula for separating 75-90% of known-sex specimens to subspecies. Hughes (1999) found that western males have significantly larger bills (p<0.001), tarsus (p<0.001), wings (p<0.05), and tails (p<0.05) than eastern males. Western females had significantly larger bills (p<0.001), tails (p<0.001), and tarsus (p<0.05), and insignificantly larger wings than eastern females.

An absolutely definitive taxonomic study has yet to be carried out. The best available scientific information, however, indicates that the western subspecies is likely valid.

Distinct Population Segment. The Endangered Species Act requires the listing of endangered "distinct population segments," regardless of whether they are recognized as species or subspecies. Distinctness is broadly defined as morphological, geographic, behavioral, genetic, or ecological separation. Morphological differences between western and eastern birds were discussed above. In addition to being significantly larger than eastern birds, live western cuckoos have darker orange bills. Live eastern bills are more lemon in color (Franzreb and Laymon 1993). Juvenile eastern cuckoos have yellow bills (Oberholser 1974), whereas the bills of western juveniles are all black until at least one month of age (Franzreb and Laymon 1993). Based on existing recordings, there also appear to be differences in song and call types between the eastern and western Yellow-billed Cuckoos (Franzreb and Laymon 1993).

Eastern and western cuckoos occupy distinct, well-defined geographic ranges on their breeding grounds in North America. The only point of contact between the two populations is a small area in western Texas where morphological differences may be clinal over a relatively short distance (Pyle 1997, Hughes 1999). The Service’s distinct population policy requires only substantial, not absolute, geographic distinction. Though less well studied, eastern and western cuckoos appear to migrate to geographically distinct areas in South America (Laymon pers. obs.; Hughes pers. comm.).

Behaviorally, western cuckoos nest significantly later than eastern cuckoos. The Service’s 90-day finding suggests that rather than indicating regional adaptation; this may be the result of a plastic species that delays nesting until local environmental factors are favorable. Though individual cuckoos can adjust their nesting date to local conditions, this plasticity is not sufficient to account for very large differences in the nesting chronology of eastern and western birds. Western cuckoos, regardless of latitude, arrive on their breeding grounds in late May and early June, not becoming common until mid-June. In the East, they arrive in late April (late March in the South) and early May. If the difference were merely the result of individual plasticity, western cuckoos should be capable nesting in late March to late April, yet they do not arrive on their nesting grounds until late May or early June. The large background difference in nesting chronology, therefore, reflects a behavioral adaptation based on genetics and geography that is only modulated by individual plasticity.

Ecologically, the nesting habitats of eastern and western populations are quite distinct. In the West, the species is entirely confined to lowland riparian woodland. It selects for sites with moist, broadleaf vegetation and avoids sites that are dominated by tamarisk (Franzreb and Laymon 1993). The western-most populations of eastern cuckoos (eastern New Mexico) make extensive use of tamarisk. Throughout the East, cuckoos nest in a wide variety of habitats including orchards, but reach their highest density in seasonally flooded mixed hardwood forest. In these forests they nest in a wide variety of tree species with no apparent selection (Wilson 1999). In California, cuckoos have a very strong preference for nesting in willows (96 of 97 nests; Laymon et. al 1997), even when cottonwoods and other tree species are present. Cuckoos in California also show a preference for nesting on east facing branches, a trait not found by Wilson (1999) in Arkansas.

Genetic analysis of eastern and western populations has not been completed. The best available scientific information indicates that western cuckoos form a distinct population based on morphological, geographical, behavioral, and ecological factors.

Significant Portion of Range. The Service’s 90-day finding asserts that the western states constitute about 27% of the cuckoo’s total breeding range. Other species such as the Marbled Murrelet in CA, OR, and WA have been listed under the ESA in similar proportions of their total range. To reduce this figure, the FWS re-calculated the range based only on riparian areas, not total regional extent. It concludes that since western riparian areas only make up 5% of all riparian areas in the continental United States, the loss of the cuckoo from the west does not endanger the entire species in a "significant portion of its range." There are numerous problems with this logic:

The most recent breeding bird survey data (BBS 2000) indicates that cuckoos, in those states east of the Continental Divide for which sufficient data exist to make statistically valid conclusions, have declined by an annual average of 1.7% between 1966 and 1998. This represents a total decline of 44.2%. Nearly all of this decline occurred between 1980 and 1998, during which time the species’ declined by an annual average of 3.1% (43.3% total decline). This disturbing trend indicates that the rate and geographic extent of the decline has increased in the second half of the previous 32 years. If the species continues to decline at 3.1% annually, it will be extirpated from large portions of eastern North America within 50 years.

Yellow-billed Cuckoos were detected in low numbers (<1.0 birds/survey route) in 16 states and one Canadian Province. In seven states, numbers were too low (<0.1/route) and/or detections too infrequent (detected on less than 10 routes) to determine a trend. Three of these were western states: Arizona, Colorado, and Montana. The remaining four were northeastern states: Maine, New Hampshire, Rhode Island, and Vermont. Declines were recorded in the remaining nine states. Significant declines were found in the western state of New Mexico; the northeastern states of Connecticut and New York; and midwestern states of Michigan and Wisconsin. Non-significant declines were found in Massachusetts, Minnesota, South Dakota, and Nebraska. No significant increases were found and the only non-significant increase was found in Ontario Province.

Yellow-billed Cuckoos were detected in moderate numbers (1.0-6.0 cuckoos/survey) in 14 states. These states were primarily in the Midwest, the Mid-Atlantic, and Southeastern regions. Of these 14 states, nine showed significant decreases (Pennsylvania, Illinois, Indiana, Iowa, Ohio, Maryland, North Carolina, Virginia, and Georgia); and three showed non-significant decreases (West Virginia, New Jersey, and Florida). Two states showed increases: Delaware where it was significant and South Carolina where it was not. The significant increase in Delaware was the only one found in any state in the Breeding Bird Survey.

Yellow-billed Cuckoos were detected in high numbers (>6.0 cuckoos/survey) in 10 states. All of these states were in the southern midwest, the southeast, and the south-central portions of the country. All tens states showed decreases: eight were significant (Kansas, Kentucky, Missouri, Alabama, Tennessee, Arkansas, Oklahoma, and Texas) and two were non-significant (Mississippi and Louisiana).

Overall, Yellow-billed Cuckoos declined significantly in 21 states. In 13, they declined significantly both from 1966 to 1998 and from 1980 to 1998. In four states (Arkansas, Iowa, New Mexico, and Oklahoma) they declined significantly from 1966 to 1998 but not from 1980 to 1998, and in four states (Maryland, New York, North Carolina, and Pennsylvania) they declined significantly from 1980 to 1998. Declines ranged from a high of 99.8% (1980-1998) in Connecticut to a low of 40.3% (1966-1998) in both Arkansas and Oklahoma. The average decline in the 17 states showing significant declines from 1966 to 1998 was 72%, while the average decline in the 17 states showing significant declines from 1980 to 1998 was 72.9%. Cuckoos are no longer detected on Breeding Bird Surveys in Connecticut and Rhode Island.

In total, breeding bird survey data shows a rather bleak picture of Yellow-billed Cuckoo population trends east of the Continental Divide. There is an evident pattern of very large population declines at the northern edges of the range, even in the face of a global warming pattern that theoretically should have made northerly regions more hospitable to the species. This northern population collapse east of the Continental Divide mirrors the temporal pattern of extirpation in the West. The species there disappeared from British Columbia in the 1920's, Washington in the 1930's, Oregon in the 1940's and Northern California in the 1950's. Its current "stronghold" in the West is in southern Arizona and southern New Mexico. Large declines have also occurred at the center of the species’ range in the Southeast and South-central states. It is alarming to see a species declining significantly at both the periphery and the center of its range.

In conclusion, the Yellow-billed Cuckoo has declined dramatically in a significant portion of its range, including many states and provinces east of the continental divide. Western declines have been most severe, with the species being extirpated from numerous states and surviving in very low numbers in other states. Though we encourage additional research, the best existing scientific data supports the listing of cuckoos west of the Continental Divide as a subspecies (C. a. occidentalis) or a distinct population segment. The listing would need to clarify how the small zone of contact in west Texas is to be handled. In addition, western North America of itself is a biologically significant portion of the species’ range. The total area of likely imperilment (west, central, and east) needs to be better quantified. However, it appears to be well over 50% of the species’ total range.


References Cited

American Ornithologists Union. 1957. Checklist of North American Birds. 5th ed. American Ornithologists’ Union, Baltimore, MD.

American Ornithologists Union. 1998. Checklist of North American Birds. 7th ed. American Ornithologists’ Union, Washington, D.C.

Banks, R.C. 1988. Geographic Variation in the Yellow-billed Cuckoo. Condor 90:473-477.

Banks, R.C. 1990. Geographic Variation in the Yellow-billed Cuckoo: corrections and comments. Condor 92:538.

BBS. 1999. Breeding bird survey data 1966-1996. U.S. National Biological Survey web page: < N.B. This page has been superceded by BBS (2000) and has been moved to the NBS archive page.

BBS. 2000. Breeding bird survey data 1966-1998. U.S. National Biological Survey web page: <http://www.mbr.nbs.gov/cgi-bin/trend98.pl>

Hughes, J.M. 1999. Yellow-billed Cuckoo (Coccyzus americanus). In. The Birds of North America, No. 418 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.

Franzreb K.E. and S.A. Laymon. 1993. A reassessment of the taxonomic status of the Yellow-billed Cuckoo. Western Birds 24:17-28.

Laymon, S.A., P.L. Williams, and M.D. Halterman. 1997. Breeding status of the Yellow-billed Cuckoo in the South Fork Kern River Valley, Kern County, California: Summary report 1985-1996. Admin. Rep. USDA Forest service, Sequoia National Forest, Cannell Meadow Ranger District, Challenge Cost-Share Grant #92-5-13.

Oberholser, H.C. 1974. The Bird Life of Texas. University of Texas Press, Austin Texas.

Pyle, P. 1997. Identification Guide to North American Birds. Slate Creek Press, Bolinas, CA.

Ridgeway, R. 1887. A manual of North American Birds. Lippincott Press, Philadelphia, PA.

Wilson, J. K. 1999. Breeding Biology of the Yellow-billed Cuckoo relative to timber harvest in a bottomland hardwood forest. Ph.D. Dissertation. University of Georgia, Athens, GA.