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Insights from Ambitious Season Studying Grassland Birds on their Wintering Grounds

By April 30, 2015Science

Rocky Mountain Bird Observatory and collaborators recently finished a third field season studying overwinter survival and habitat use of Baird’s and Grasshopper Sparrows in the Chihuahuan Desert grasslands of northern Mexico. It was an ambitious season: Across the three sites in Chihuahua, Durango and Coahuila, dedicated teams captured, radio-tagged and tracked 120 Baird’s Sparrows and 135 Grasshopper Sparrows, amassing thousands of location points over three-and-a-half months.

Carcay

Reserva Ecológica El Uno and Cerro Carcay form a majestic backdrop to the Chihuahua field site. Photo by Greg Levandoski.

 
Coahuila

With the addition of a third site in Valle Columbia, Coahuila, Mexico, to the study area this winter, we are gaining an even broader perspective on overwinter survival, movements and habitat preferences in declining grassland birds. Photo by Héctor García.

 
A Summary of the Data

One of our primary goals is to determine mortality and survival rates of these birds. Birds in Durango experienced the highest mortality rates, with 66 of 127 radio-tagged sparrows there killed by predators, such as Loggerhead Shrikes and American Kestrels. Only 7 of 39 birds and 25 of 91 birds were killed at the Coahuila and Chihuahua sites, respectively.

At the Chihuahua site, where birds have been monitored for three winters, we’ve seen a lot of variation in apparent survival. We consider a bird to have survived if we recaptured it at the end of the season or if it was observed alive through expected transmitter life (50-60 days). In Chihuahua, about 26% of radio-tagged birds survived the very cold and wet winter of 2012-13, compared with about 53% of radio-tagged sparrows surviving in 2013-14, a warmer winter with better grass cover and higher bird densities. This season, which was generally cooler than last winter, fell in the middle with 42% of radio-tagged birds surviving. Preliminary results from the first two seasons in Chihuahua suggest cooler temperatures are detrimental to survival. Further analyses may elucidate whether factors such as dense grass, which provides thermal refuge for sparrows and cover from predators, or taller shrubs, which act as perches for predators, influenced this season’s survival rates.

As with any telemetry study, a proportion of birds go missing or we cannot explain their fate. Across the three sites, 38% of individuals went missing or had unknown fates. It is unclear what happened to these birds, although it is possible many of them left the study area to find new territories. Preliminary analyses of winter 2012-13 and 2013-14 data suggest that a proportion of the wintering population makes forays to new territories or wander widely across the study area. Thus, some individuals may have moved out of our range of detection. We attempt to age all birds and collected feathers to genetically determine sex to see if this can explain differences in movement patterns.

Tracking a Lone Pipit

In addition to the Baird’s and Grasshopper Sparrows, we tracked a lone Sprague’s Pipit for 45 days to gather data on movement and habitat preferences. These data are extremely novel and valuable as this is the first radio-tagged Sprague’s Pipit to be tracked on its wintering grounds! On our last day in the field, employing persistence, stealth and creativity, we recaptured this individual and removed its transmitter. We now have new insight into how we can advance the study of this rare and declining species and aim to capture and track other Sprague’s Pipits in subsequent winters.

SPPI

A Sprague’s Pipit, a cryptic species that breeds in the northern Great Plains and winters in the Chihuahuan Desert, shows off its camouflage. After tracking one individual this winter, we have an even better idea of how and where to capture more individuals of this species. Our pipit spent most of its time in areas with sparse grass and bare dirt and was often found near other pipits. Photo by José Hugo Martínez Guerrero.

 
Plow

A tractor plows once pristine grassland near the Chihuahua study site. Unregulated and rapid conversion of grasslands to agriculture is destroying habitat critical to the persistence of countless grassland birds. Photo by Ben Swecker.

 
GRSP

Grasshopper Sparrows are banded, measured and weighed before being outfitted with transmitters. This season, we recaptured 10 sparrows that had been banded previously. Three of the recaptured birds even wore transmitters in prior winters and returned to the same territories. It’s difficult to fathom how such a diminutive bird can make the round-trip journey year after year! Photo by Erin Strasser.

 
Deer

Plenty of non-avian wildlife can be spotted at the study sites including these mule deer, javalina, rattlesnakes, black tailed prairie dogs and badgers. Photo by Héctor García.

 
Sunset

Our team in Chihuahua is lucky enough to spend each day working under the vast and striking Chihuahuan Desert sky, and we never get tired of beautiful sunsets! Photo by Erin Strasser.

 
Working Across the Full Annual Life Cycle

One of our goals is to explore the full annual life cycle of these species, pinpointing where and why populations are crashing and identifying geographic connectivity between the breeding and wintering grounds. This spring and summer, we begin a similar project on the breeding grounds in North Dakota to gain an understanding of breeding season survival in adults and juveniles. We’ll track adult and juvenile birds using radio-telemetry and will deploy geolocators to find out where they spend their winters. Who knows, maybe we’ll encounter some of these birds down in Mexico next winter!

Thank you to Universidad Juárez del Estado de Durango, Universidad Autónoma de Nuevo León and The Nature Conservancy for partnering on this research, and the Canadian Wildlife Service, USFWS Neotropical Migratory Bird Conservation Act, USDA Forest Service International Programs and WWF-Carlos Slim Alliance for funding.

Read a prior update from this study season, as well as results from the 2012-13 season and results from the 2013-14 season of this study. We’re already looking forward to season four next winter!

~ Erin Strasser, Biologist