Lucy’s Warblers are small, desert-dwelling warblers found in the Southwestern United States. These warblers have historically found their stronghold in dense, lowland riparian mesquite bosques, which can be some of the hottest places in the country. They can also be found nesting in the bark of Joshua Tree yucca in the Mohave desert, in old woodpecker-excavated cavities in Saguaros, and in senescing cottonwoods.

Despite the extreme heat, Lucy’s Warblers have developed strategies for survival in the desert. First, they are among the earliest migrants, arriving at their breeding grounds in early March and departing by late June or early July. This timing allows them to avoid the peak heat of summer. Second, they construct their nests in well-concealed cavities or behind large flakes of mesquite bark, providing protection for their eggs and chicks from the harsh desert sun. Interestingly, Lucy’s Warblers are the only western warbler that have been documented nesting in cavities.

Lucy’s Warblers feed almost exclusively on insects and arthropods during the breeding season. A 1996 study by Poulin and Lefebvre revealed that Lucy’s Warblers take advantage of whatever prey is most abundantly available (often ants, beetles, and true bugs), but they prefer highly nutritious spiders and moth larvae to feed their young in the spring.

These diminutive warblers were considered to have a declining population in the late twentieth century due to habitat loss. Specifically, the diversion of groundwater and surface water from natural riparian areas and the over-harvesting of mesquite for firewood dramatically reduced historic breeding habitat. When we combine this water scarcity and fragmentation of mature mesquite bosques with urban expansion, Lucy’s Warblers are left scrambling to find alternative nesting habitat. This inspired Tucson Audubon Society to initiate a nest box project for Lucy’s Warbler. They studied the nest structure in mature mesquites and developed several different nest box designs. After years of monitoring, they arrived at a simple triangle-shaped box. Boxes placed in young mesquite in Tucson are now being used by Lucy’s Warbler- a wonderful way to provide breeding habitat in developed areas.

Population trends modeled by eBird from 2012-2022 show an increasing population trend. Efforts to protect our riparian areas and the promotion of using native plants in urban landscaping have likely been contributing factors to this rebound. The San Pedro River in Arizona is a great place to observe these birds, as it has many miles of mesquite bosques that have some of the highest densities of breeding Lucy’s Warbler.

Despite the resilience of this small warbler, the challenges it will encounter in the coming decades are formidable. Audubon’s Survival by Degrees project has harnessed millions of bird observations and combined them with sophisticated climate models to forecast the impact of climate change on North America’s bird ranges. With a predicted 3-degree Celsius temperature increase, the results for Lucy’s Warblers reveal a 37% loss in habitat along the southern borders of Arizona, New Mexico, and California, but a 170% range expansion on the Colorado Plateau and in the southern part of the Great Basin. However, it’s essential to recognize that these models, no matter how sophisticated, remain predictions. In an unpredictable future, it is impossible to account for all factors influencing dispersal, recruitment, and survival. How will an extended migration north impact the timing of their breeding? Will Lucy’s Warblers find sufficient food resources in Nevada or Colorado? And will they face increased competition from other songbirds already inhabiting the northern fringes of their range?

Our Director of Bird Conservation, Tice Supplee, recently shared a fond memory of being in a mesquite bosque during spring migration when Lucy’s Warblers literally fell from the sky to rest in the shade of the trees:

“My most memorable encounters with Lucy’s Warblers take place in a very different setting – a cottonwood gallery forest of the Rio Grande in Albuquerque. If you’ve taken a stroll along the middle Rio Grande in the past ten years during the spring, you’ve undoubtedly heard the bouncy trill of a Lucy’s Warbler song ringing from the treetops. The first time I heard them I was stopped in my tracks… what is that?! A Yellow Warbler? No… a migrating Virginia’s Warbler? No… I knew it sounded familiar, but I couldn’t put my finger on it. After several minutes of craning my neck towards the sky and performing what must have seemed like a neurotic dance of scurrying around the bosque while I balance my binoculars on the bridge of my nose, I finally spotted it! Lucy’s Warbler! Are those typically seen here? After talking to some local birders, I learned that they have been showing up in Albuquerque more frequently. Biologists at Rio Grande Bird Research Inc., who have been banding birds in the middle Rio Grande for decades, encountered their first Lucy’s Warblers in 2018 at Valle de Oro National Wildlife Refuge, where they captured 12 young birds. Two years later, they began capturing them in mist nets at the Rio Grande Nature Center in Albuquerque. Now, they consistently capture both young birds and adults every year at both sites.”

Change is upon us – let us work together to better understand how the world is changing so we can solve tomorrow’s problems. So get out there and find some birds on the frontiers of their range! Scramble up the desert washes, dance in the bosques, wade through the mud, and keep birding!