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Tucson Audubon Society
Critical Riparian Bird Species Monitoring:
Villa Verde, Sonora, México
by Jennie Duberstein, Proyecto Corredor
Colibrí
(Reprinted from the May-June 2004 issue of the Vermilion Flycatcher newsletter)
In
the summer of 2003 Proyecto Corredor Colibrí (PCC) received grants from T
& E, Inc. and the Sonoran Joint Venture to implement Southwestern
Willow Flycatcher and Yellow-billed Cuckoo surveys at Villa Verde, a main
tributary to the Mexican Upper San Pedro River. The goal of the project
was to provide baseline data for long-term bird monitoring efforts at
Villa Verde and in the rest of the Upper San Pedro Watershed in Sonora.
Because birds and ecosystems do not recognize international boundaries,
this project also helped to provide a more complete picture of the health
of cuckoo and flycatcher populations on the entire Upper San Pedro River,
on both sides of the border.
The San Pedro River begins in northern Sonora, Mexico and flows north before crossing the international boundary and continuing into southeastern Arizona. In 1988, a forty-mile stretch of the river, from the international border north, became the San Pedro Riparian National Conservation Area. Between one and four million migrating songbirds use the San Pedro River each year. At the San Pedro Riparian National Conservation Area (SPRNCA), over 375 bird species inhabit the San Pedro during the year, and at least 100 species are known to breed there. The American Bird Conservancy and the Commission for Environmental Cooperation recognize the San Pedro River as the first Globally Important Bird Area in North America, and the American Rivers Organization listed it among the ten most endangered rivers in the United States. Simply put, the Upper San Pedro Watershed is one of the most biologically important corridors in the western hemisphere.
Despite this, much of the conservation work in the watershed stops at the U.S. border, and areas located just a few miles south remain almost unknown. Located approximately 15 miles southeast of the SPRNCA and the U.S. border, in Sonora, Mexico, is Villa Verde. It’s located between two ejidos, or small, cooperatively run communities, where ranching is the main source of income. Its large reservoir and six mile stretch of riparian habitat support three of the most endangered habitats in the San Pedro riparian ecosystem with cottonwood/willow gallery forest, ciénega, and sacaton grassland. When these three are combined with the diversity of other habitats at Villa Verde, the area supports a great variety of bird species. To date, we have documented 190 species there, and the list continues to grow.
Yellow-billed Cuckoo (Coccyzus americanus) historically bred throughout most of western North America, but populations have declined dramatically in the last 80 years due primarily to habitat loss. For the past three years, the Southern Sierra Research Station has conducted a Yellow-billed Cuckoo study on the Arizona side of the Upper San Pedro River. PCC staff members worked as field technicians on this project in the summers of 2002 and 2003, becoming familiar with survey techniques and methodology, as well as Cuckoo biology.
The Southwestern Willow Flycatcher (WIFL, Empidonax traillii extimus) is an endangered species in the United States whose populations have been in steady decline due to habitat loss. While intensive WIFL surveys are being conducted in Arizona, little is known about the Sonoran population of this species. Villa Verde provides the best potential habitat on the Mexican side of the watershed for hosting breeding WIFLs, and one of the best areas on the Upper San Pedro on either side of the border. Especially because of Villa Verde’s close proximity to the SPRNCA and the Arizona border, knowledge about Mexican populations is critical to gaining a better understanding of the status of WIFLs in the region.
Results
We conducted four rounds of Cuckoo surveys between 5 June and 19 August 2003. PCC is currently implementing riparian habitat restoration projects at two of the survey sites, and these surveys will provide baseline data about the effects of our restoration efforts on Cuckoo and Willow Flycatcher populations. Whenever possible, we surveyed the different segments either on the same day or within the same week.
For Cuckoos, the high count came in mid-June, when we counted fourteen individuals, or seven pairs of birds. The count declined in July, but increased again in August, leading us to believe that birds simply weren’t responding to our July tape surveys. Early in June birds may still be arriving from their wintering grounds in the south, but by the end of June to mid-July most Cuckoos in southeastern Arizona and northwestern Mexico are on their breeding grounds. Keeping this in mind, we estimate anywhere from three to seven pairs of Yellow-billed Cuckoos utilizing the six mile stretch of Villa Verde.
Although we searched for Cuckoo nests, we were unable to locate any. Based on the numbers of Cuckoos observed and the dates of observation, however, we believe Cuckoos are nesting at Villa Verde, despite the absence of physical evidence.
We conducted nine Southwestern Willow Flycatcher (WIFL) surveys at Villa Verde between 5 June and 8 July 2003. We failed to detect any WIFLs, although a Cuckoo responded to the flycatcher tape on one survey. While we didn’t observe any breeding WIFLs, during a birding tour to the area on 9 August 2003, birders spotted a Willow Flycatcher. A subsequent group on a birding tour on 12 August also observed two individuals, and so on 14 and 15 August 2003 we returned to conduct another survey to attempt to determine whether or not WIFLs were using Villa Verde during migration. Although the birds did not vocally respond to the tape, Willow Flycatchers immediately and consistently flew in to the surveyor to investigate the taped call (seven individuals on 14 August and two individuals on 15 August). Due to the time of year, however, we cannot definitively identify these birds as the Southwestern (extimus) subspecies of Willow Flycatcher.
The work supported by T & E, Inc. and the Sonoran Joint Venture has added a new dimension to our understanding of bird populations at Villa Verde. We have increased our familiarity with two little-known species and can add this information to the existing body of knowledge in the region. Villa Verde continually proves to be an important oasis for birds as both breeding and migratory habitat.
If you would like to learn more about Proyecto Corredor Colibrí and our work in Sonora, including how you can visit Villa Verde, please visit our website at www.p-cc.org, contact us at info@p-cc.org, or call 520-432-9116.
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This page was updated on 12/28/05