Make the trees tall, thick and dense, let the water run free -- and the cuckoos will come. 

That’s a big part of what the rare, potentially protected Western yellow-billed cuckoos needs in the way of living space, says a researcher who has studied the elusive birds for 20 years.

“They require native (riparian) habitat – cottonwoods and willows, tall trees, 15 to 40 feet tall,” said Matt Johnson, who works for Northern Arizona University and also has researched the cuckoo for the United States Geological Survey.

Also important is that the treetops have thick, dense canopy cover of tree foliage up high, said Johnson, director of NAU’s Colorado Plateau Research Station. This is important for many birds, not just cuckoos, to hide nests from predators, and to moderate daytime temperatures, shielding the birds from the brutal desert sun, said Christopher Calvo, a research biologist who works with Johnson at the Colorado Plateau station.

The U.S. Fish and Wildlife Service recently proposed to list the cuckoo as a threatened species, potentially triggering a suite of protective measures if the bird is listed – or a huge conflict pitting environmental interest groups against development and other business interests over whether it should be listed.

While another troubled Southwestern bird, the endangered Southwestern willow flycatcher, has thrived in non-native tamarisk trees that are slowly taking hold across many rivers in this region, the cuckoos are choosier. Their favorite riparian groves consist of native cottonwood and willow trees up high, with what’s known as “sub-canopy” layers of tamarisk or willow down below, Johnson said.

In such circumstances, cuckoos will even nest in tamarisks, he said. But they don’t usually occupy areas where the invading tamarisks are the predominant or only major trees present.

He and other researchers have had similar results tracking cuckoo habitat on the Lower Colorado River, the Bill Williams River in western Arizona, the Lake Mead Delta, the San Pedro River and the Verde River since the early 2000s. The cuckoo also is seen frequently by birdwatchers in similar habitat on the Upper Santa Cruz River, especially near Rio Rico and Tubac, Calvo said.

In 2009, while Johnson worked for U.S.G.S., the agency published a brief report saying that the cuckoos found on the Bill Williams River, for instance, did not breed in small, isolated patches of riparian vegetation or in tamarisks.

Other researchers found at the San Pedro and Verde rivers concluded that cuckoos were found most often in riparian areas bordered by large stands of mesquite bosque groves. Many researchers consider these mesquite buffers to be integral parts of the riparian areas, and it’s often the mesquite bisques where the majority of cuckoo foraging takes place, Calvo said.

To learn more about the birds’ climate preferences, U.S.G.S. researchers measured temperature and humidity occupied and unoccupied sites along the Lower Colorado River. The occupied sites were consistently cooler during the day and more humid both day and night than the unoccupied sites, the researchers found.

The birds are so shy, secretive and reclusive that often, its loud, “ka, ka, ka, ka, ka, kow, kow kow” is often the only evidence of its presence, U.S.G.S. wrote in 2009. The birds arrive in their Southwestern breeding range from the South American winter homes in early June, and breed here until late August.

From Johnson’s view, one of the biggest problems plaguing the cuckoos in the U.S. is that over the last three or four decades, the dams built during the 1950s, ‘60s and ‘70s across many Southwestern river corridor wiped out much of their historic cottonwood-willow breeding grounds.

Other main causes of riparian habitat loss are the conversion of these areas for farming and other uses, water diversion, stream channelization projects to straighten the rivers and livestock grazing, scientists have said. Groundwater pumping and non-native plant invasions have also taken their toll on streamside plant life.

“All these areas with dams just became a canal. They lowered the water table so low that the cottonwoods and willows can’t tap into it,” Johnson said.

Plus, it’s unknown what, if any problems the bird has encountered in its South American wintering grounds, Johnson said. They spend seven to eight months annually down there. But due to lack of research, researchers aren’t sure what habitat they occupy and how much other factors such as pesticides may affect the birds, he said.

“Are they losing habitat? Are they greatly affected by pesticides on their winter grounds? Who knows?” Johnson said.

What researchers have learned is that habitat changes can clearly affect cuckoo populations. Because the Lower Colorado River, for instance, is an area that has been part of a federal habitat conservation plan, authorities have started restoring some of the river’s vanished cottonwood-willow stands over the past two decades.

Yellow-billed cuckoos have started breeding in those restoration sites in the past five to six years, including areas south of Cibola National Wildlife Refuge north of Yuma, Johnson said. The same phenomenon has occurred at the Palo Verde Ecological Reserve just north of Cibola and at the Ahakhav Tribal Preserve along the Lower Colorado outside the riverfront town of Parker, he said.

At the same time, at the Lake Mead Delta where the Colorado River flows into the lake near the Nevada border, surveys conducted in 2006 found many breeding cuckoo pairs, but a year later found none. What happened? The region’s continuing drought had killed off many of the native trees where cuckoos had bred in 2006 – the same drought that had dropped the lake’s water level by 15 feet over the same period.