New breeding habitat discovered for rare Yellow-billed Cuckoos

FLAGSTAFF, Ariz. - The western population of yellow-billed cuckoos (Coccyzus americanus occidentalis) was thought to nest only in mid-elevation riparian areas dominated by cottonwood and willow trees, with perennial surface water.

A new paper published in the Journal of Field Ornithology documents, for the first time, the wide extent of their occurrence and nesting in arid habitats in foothill, mountain, and lowland drainages of the "Sky Islands" in southeast Arizona, and their use of arid drainage tree species such as oak, mesquite, hackberry, juniper, ash, and acacia - a dramatic difference to their cottonwood/willow "typical" streamside habitat.

While cuckoos had been thought to occupy dry, hot areas on occasion, previous surveys had not identified extensive use or nesting there.

Yellow-billed cuckoos once ranged all the way from British Columbia south to Mexico and have declined by over 50% in the last 45 years. Their remaining stronghold in the U.S. is primarily in Arizona and New Mexico.

Previous studies in the southwest U.S. mostly focused on riparian areas. The most recent extensive surveys by the U.S. Geological Survey and cooperators documented 400-425 cuckoos in Arizona, almost all along rivers and perennial streams.

Detecting yellow-billed cuckoo nesting habitat is challenging. They are rather secretive birds with large home ranges, infrequent calls, concealed nests, and a nomadic period before nesting when they travel widely to assess prey availability, such as caterpillars, insects, and wild fruits at potential nesting sites. This makes it difficult to determine whether they are passing through or nesting in an area.

Yellow-billed cuckoos also are late nesters. Unlike many birds that breed in the spring, yellow-billed cuckoos nest from May to September, with peak nesting in July and August - the hottest months in the desert. The newly identified xeric nesting areas are places that are less used by hikers and birders during these months. However, anecdotal reports of cuckoos in oak woodlands, and documented sightings by birders recorded on the online eBird platform spurred further analysis of cuckoos in arid southeastern Arizona locations, leading to this study.

The study surveyed these "xeroriparian" sites over a four-year period using a standard U.S. Fish and Wildlife Service -accepted survey protocol. The team of researchers included the USGS, USFWS, Northern Arizona University, U.S. Forest Service, and the Audubon Society. The study areas included major mountain ranges and foothills between the San Pedro River and the Baboquivari Mountains, with additional historic data collected east of the San Pedro River.

They found nesting cuckoos mostly in the western portion of the Sky Islands, particularly in the region extending from the Baboquivari Mountains in the west, east through the Altar Valley, Atascosa Highlands, Santa Rita Mountains, Patagonia Mountains, Canelo Hills, San Rafael Valley, and western side of the Huachuca Mountains. They observed fewer cuckoos in the Whetstone Mountains, Rincon Mountains, and Santa Catalina Mountains.

The numbers of nesting pairs found by this study in novel arid habitats - 100 new sites, including many with multiple nests or territories - are a significant addition to the known population in the western U.S., says USGS research zoologist Charles Drost. And researchers found that population numbers of breeding pairs in these dry foothill and mountain canyons are comparable to numbers in the generally recognized breeding habitat of lowland, gallery riparian forests.