Field Season Wraps Up in Mexico: Why Was Sparrow Survival Higher This Winter?

By April 23, 2014Science

Longer days, warm and sometimes ferocious winds, rattlesnakes emerging from hibernation, and cacophonous flocks of migrating Snow Geese signaled the advent of spring and the end of RMBO’s winter field season in the grasslands of northern Mexico. In early March, after three months tracking winter survival and habitat use of Grasshopper and Baird’s Sparrows, we recaptured sparrows and removed their transmitters.

From December to early March, using radio-telemetry, we tracked 74 individual birds at our study site in Chihuahua and 47 birds at our study site in Durango. Daily tracking allowed us to determine rates and causes of sparrow mortality. In Chihuahua, 7 sparrows (9.5% of radio-tagged birds there) were killed by predators. In Durango, 7 sparrows (14.9% of radio-tagged birds there) were killed by predators. At both sites, the principal predators were Loggerhead Shrikes; however, American Kestrels, Merlins and Northern Harriers were commonly observed patrolling the areas.

Our observations suggest that there are dramatic differences in survival between years and highlight the importance of multi-year investigations. Last winter, 33% of sparrows at the Chihuahua site were killed by predators! Had our research been limited to the most recent season, it might appear winter survival is relatively high and not a factor in grassland bird population declines. We’ll have to delve into the data to understand why survival was higher this winter, but the lack of snowstorms, taller, denser grass following heavy summer rains, and higher sparrow densities probably contributed. Research will continue this upcoming winter and will likely include a third site.


Map of the study areas in northern Mexico: Reserva Ecológica El Uno in Chihuahua and Rancho de Santa Teresa in Durango.


The Chihuahuan Desert grasslands are an imperiled and starkly beautiful ecosystem. The vast majority of grassland birds that breed in the Great Plains of the U.S. and Canada winter in this region. Photo by Erin Strasser.

Grasshopper Sparrow

A Grasshopper Sparrow wearing a radio-transmitter is captured in a mist net. In March at the Chihuahua site, the 12-person team recaptured 35 of the 38 remaining radio-tagged sparrows. Birds were recaptured to assess condition and remove transmitters, easing their northward migration to the breeding grounds. Photo by Denis Pérez.

Shrike Kill

An unfortunate Grasshopper Sparrow meets its fate with a Loggerhead Shrike. Shrikes impale their prey, which include birds, rodents, reptiles and insects, to spines or barbed wire fence for easy eating. Photo by Denis Pérez.


Difficult times call for different measures. After many failed attempts to capture a sparrow, the group tries a new tactic: closing in on the radio-tagged bird with three nets. Photo by Veronica Flores Diaz.


Birds wait to be banded. Between two field sites, we banded 281 Ammodramus sparrows (224 Grasshoppers and 72 Baird’s). Despite banding almost 200 birds last year, we only recaptured one bird banded in a previous season. Photo by Denis Pérez.

Sideoats grama

Sideoats grama (Bouteloa curtipendula) appears to be one the preferred foods of wintering Grasshopper Sparrows. RMBO has partnered with a Ph.D. student from the Autonomous University of Chihuahua to better understand winter diet of Grasshopper and Baird’s Sparrows. Photo by Erin Strasser.


Measuring bill size can give us information on size variation and insight into habitat partitioning. Do Grasshopper Sparrows with larger bills prefer grass with larger seeds, thus reducing competition with the smaller-billed Baird’s Sparrows? Photo by Jóse Hugo Martínez.


Those are a lot of numbers to memorize! With up to 50 uniquely numbered radio-tagged birds flying around at any given time, the team had to keep a checklist. Photo by Erin Strasser.


School groups benefit from visits to study sites. Not only do they learn about bird and grassland conservation, they help us round up the sparrows. Photo by Erin Strasser.


What’s doing bird research without assessing habitat as well?! Technicians Mariana and Chuy determine vegetation characteristics within a bird’s territory. This information can help us identify what habitat characteristics are important for survival, and inform management practices that conserve or create favorable habitat. Photo by Denis Pérez.

Black-throated Sparrow

Black-throated Sparrows are a stunning addition to the Chihuahuan Desert sparrow assemblage. Photo by Erin Strasser.


Baird’s eye view: Some individuals appear to prefer sites with bare ground interspersed with dense patches of grass. Grass seeds accumulate at these “borders.” Photo by Erin Strasser.


Team Ammodramus is ecstatic after recapturing 35 of 38 radio-tagged birds remaining at the end of the study season. Photo by Denis Pérez.

Two weeks after wrapping up our winter study season in Chihuahua, I returned to an eerily quiet field site; the migratory birds had begun their northward journey. Missing were the tumultuous swarms of Chestnut-collared Longspurs, and only a few Grasshopper Sparrows buzzed in alarm as I walked through the site. It reminded me how much more magnificent the grasslands are with their bird counterparts, and that I couldn’t conceive of them any other way.

Interested in learning more? Read more posts about this research.

Thank you to Universidad Juarez del Estado de Durango, Universidad Autónoma de Nuevo Leon, Universidad Autónoma de Chihuahua and The Nature Conservancy for partnering on this research, and the Canadian Wildlife Service, USFWS Neotropical Migratory Bird Act, USDA Forest Service International Program, National Park Service and Commission for Environmental Cooperation for funding. And thank you to all the volunteers who helped us capture birds!

~ Erin Strasser, Biologist