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Broadcast on September 16, 2010
By Colin Shackelford
As the days get shorter and the temperatures begin to drop, you may begin to notice flocks of “little brown birds” beginning to arrive. Today, we’ll explore the link between the grasslands of the Chihuahuan Desert region and our little brown winter guests.
Wintertime brings an abundance of migrating sparrows, pipits, longspurs and other grassland birds to the grasslands of the Chihuahuan Desert region. In the fall, these birds fly south from the western great plains of the northern United States and southern Canada seeking a safe haven to wait out the snows and cold temperatures of their breeding grounds in the north.
Ninety percent of the migratory grassland bird species spend the winter in the Chihuahuan Desert grasslands of northern Mexico and the southwestern United States. These grasslands, occupying only about 12% of the total area of the region, are vitally important for the survival of millions of North American grassland birds.
Grassland birds were historically found in vast numbers across the prairies of the western Great Plains. Today, grassland birds have shown steeper, more consistent, and more geographically widespread declines than any other group of North American bird species. Many grassland birds are undergoing massive population declines due to large-scale, on-going habitat loss and degradation over much of their range. As an example, the entire North American population of Grasshopper Sparrows has declined by more than 50% over the last 30 years. Threats to native grasslands include expanding agriculture, urbanization, desertification, and the expansion of invasive plant species such as bufflegrass.
The conversion of Chihuahuan Desert grasslands to cropland has recently begun to accelerate in Mexico—especially in the state of Chihuahua. Since 2005, nearly 1 million acres in Chihuahua have been converted to irrigated agriculture. This recent surge in agricultural conversion will significantly impact the overwintering habitat for many grassland birds over the coming years.
Grasslands also face another threat – shrub encroachment and desertification. Changing climate patterns, inappropriate levels of historic grazing, and the suppression of naturally occurring fire have set the stage for the invasion of grasslands by shrubs such as juniper, mesquite, cat claw mimosa, and other woody plants. These invading shrubs change both the structure and composition of the grasslands.
This is a problem since many grassland birds have very specific habitat needs. As changes in plant composition and structure alter the types of habitat available for birds, the species of birds overwintering there may also change. For example, Chestnut-Collared Longspurs prefer to overwinter in open grassland with a low number of shrubs. As shrubs invade, birds such as Black Chinned Sparrows move in, and the Chestnut-Collard Longspur must seek another patch of open grassland.
For several years, the Rocky Mountain Bird Observatory has been conducting a census of winter grassland birds in northern Chihuahua. Their research shows that roughly 35% of their grassland study sites now have shrub cover greater than 3%. Though this seems like a very low number it represents a threshold above which habitat is no longer suitable for many species of grassland birds. The Sprague’s Pipit—a bird that requires open grassland with very little shrub cover—has been particularly hard hit. The Observatory’s census data show an 8-fold decrease in the Sprague’s Pipit population.
Because grassland birds are migratory they are doubly at risk as changes occur in their breeding habitat in the northern Great Plains and in their wintering habitat in the Chihuahuan Desert grasslands. Annual breeding bird survey data collected across North America by the Patuxant Wildlife Research Center has shown that the declines in the numbers of breeding grassland birds in their summer habitat in the northern Great Plains parallel the losses of habitat taking place in their Chihuahuan Desert winter habitat.
Even though it has been well documented that Chihuahuan Desert grasslands are globally important to many grassland birds, especially in winter, there is still very little scientific information on wintering grassland bird distribution, habitat use, and seasonal movements to help guide conservation in this region. A new research partnership between the Rocky Mountain Bird Observatory, the Borderlands Research Institute for Natural Resource Management at Sul Ross State University, the National Park Service and the Sonoran Joint Venture will help shed light on some of these unknowns. Part of this research partnership involves expanding winter bird surveys and monitoring into parts of west Texas, southern New Mexico, southern Chihuahua, and northeast Sonora. One of the new monitoring sites include the grasslands found at the Chihuahuan Desert Nature Center. This new research will help to better inform and prioritize conservation decisions and restoration efforts in the Chihuahuan Desert Region.
Have a question or comment about this episode? Contact Nature Notes Coordinator Megan Wilde at mwilde [at] cdri [dot] org. Or discuss this episode on Nature Notes’ Facebook page. This episode originally aired on September 16, 2010.
References & Resources for Educators
- Rocky Mountain Bird Observatory
- Sonoran Joint Venture
- Borderlands Research Institute at Sul Ross State University