Canyon Sketches Vol 08 - December 2008 Park Biologists Survey for Non-Native Brown-Headed Cowbirds

Park Biologists Survey
for Non-Native Brown-Headed Cowbirds

by Brian Gatlin, Park Ranger - Interpretation


Available as an audiocast read by Patrick Gamman. Duration 5:54 (4.72MB) Play Now

cowbird egg found in another bird's nest.
Cowbird egg in a chipping sparrow nest.
Throughout the spring and summer nesting season, biologists from the Division of Science and Resource Management at Grand Canyon National Park intensively searched for bird nests at several locations within the park. When they located the nests, they carefully examined the eggs within the nests. Often, they saw a single egg that was noticeably larger than the others. This was the egg of a brown-headed cowbird (Molothrus ater).

When cowbird eggs were located, the biologist carefully removed them and shook them vigorously before replacing them in the nest. This was done with the hope that these addled eggs would not hatch. The biologist then made careful note of the nest location and the type of bird that had made the nest.
Interfering with the reproductive success of wildlife is not a typical day’s work for a National Park Service biologist. But these biologists are part of a three-year program designed to reduce the numbers of brown-headed cowbirds at the Grand Canyon while learning more about their impacts.

Trapping or removing one wildlife species to protect another is not a new idea. In the early 1900s, many national parks and forests had predator control programs. For example, to preserve deer, resource managers and game wardens shot mountain lions—with predictably disastrous ecological results. Deer populations exploded, exceeding the land’s carrying capacity and leading to mass die-offs. But unlike these early programs, which disrupted functioning ecosystems, today’s cowbird control program is aimed at restoring a natural balance.

Brown-headed Cowbird (Female) Molothrus ater
Brown-headed Cowbird (Female)

USFWS Photo by Lee Karney

Cowbirds are not native to Arizona. Originally restricted to the open grasslands of the Great Plains, cowbirds have followed modern human impacts across North America, colonizing new landscapes wherever cattle, agriculture, or suburban landscapes have spread. First seen in Arizona in 1934, brown-headed cowbirds are now one of Arizona’s most common birds.

Brown-headed cowbirds are obligate parasites, meaning that they rely on a host of another species to bring up their young. Instead of building nests, incubating their eggs, and raising their young, cowbirds locate the nest of another bird, lay their eggs there, and leave them for the other bird—the host—to raise for them. Cowbird eggs typically hatch sooner, and cowbird chicks are generally larger than those of the host species. This allows the cowbird chick to dominate the others and receive more food from the host parents. In the battle for survival in the nest, cowbird chicks usually win.
Cowbirds lay their eggs in the nests of many different species.
Cowbird egg in a Plumbeous Vireo nest.
Cowbirds lay their eggs in the nests of many different species. One of them is the endangered southwestern willow flycatcher (Empidonax traillii extimus). While cowbirds have expanded their numbers, following humans and our impacts, willow flycatcher populations have plummeted for related reasons. Willow flycatchers were once common along rivers and streams throughout the southwest, but much of their former habitat has been flooded by dams along these rivers, while other habitat has been lost to urbanization or water diversion projects. At the same time that these flycatchers’ breeding habitat disappeared, cowbirds have expanded into their range and begun to parasitize their nests. Southwestern willow flycatchers were listed as endangered in 1995, and the population today is approximately 1000 pairs.
The southwestern willow flycatcher breeds within the Grand Canyon along the Colorado River. Between 1982 and 2002, half the willow flycatcher nests found within the canyon had been parasitized by cowbirds. Cowbirds also impact other native species in the park. Park wildlife biologist Rosa Palarino says, "Cowbirds have been identified as one of many important factors contributing to the decline of several songbird species."
Mirroring a gnatcatcher nest.
Mirroring a gnatcatcher nest.
In 2007, Grand Canyon National Park launched the cowbird research program to measure the impact of cowbirds on the endangered southwestern willow flycatchers and other species. The cowbird control program is designed to monitor and record cowbird abundance and concentration points within the park, and to identify the species that they parasitize.

In 2008, biologists conducted intensive nest searches in Grand Canyon Village, as well as inner canyon locations such as Indian Gardens and Phantom Ranch. By moving quietly and carefully observing bird behavior, they were able to locate over 500 nests. Once the nests were located, biologists used wireless cameras and long poles with mirrors on the end to see inside nests without actually climbing up to them. When cowbird eggs were discovered, they were shaken, or addled, and cowbird chicks were removed from nests. Adult cowbirds were also trapped and removed from the park.
Nest peeping.
Nest peeping
Palarino said, "It is our hope that this project will not only reduce the number of brown-headed cowbirds at Grand Canyon National Park, but that we will also gain a greater knowledge of their impacts on our native bird species.

The data gathered will help us better understand the habits and distribution of cowbirds within the park, and may prove essential to the survival of the local population of southwestern willow flycatchers as well as other native birds.”
As human impacts affect more and more of the planet, projects like Grand Canyon’s cowbird study will become more necessary. While populations of species like cowbirds expand because of human presence and associated impacts on natural environments, other, wilder native species disappear. If park managers hope to preserve these wild and vulnerable species within intact and functioning ecosystems, the intervention of biologists may be one key to their success.

For more information on brown-headed cowbirds:

For more information on southwestern willow flycatchers:

Final Recovery Plan: Southwest Willow Flycatcher - August 2002

Last updated: February 24, 2015

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