Field Notes
U.S. Fish & Wildlife Service
Field Notes Entry   
Service Joins Army to Protect Endangered Bird Habitat on Fort Hood, Texas
Southwest Region, February 22, 2001
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If Steven Spielberg ever required another set location to shoot his next epic war picture, Fort Hood, Texas would likely serve well. As one of the largest heavy artillery training sites in the country, Fort Hood conducts live weapons fire and aviation training and houses 544 armored tanks. Built to destroy and engineered to withstand severe combat, tanks such as the M1-A2 Abrams boast such features as guided missile launchers and Global Positioning Systems (GPS), and they weigh between 50 and 80 tons. Paved roads can buckle and crumble apart like tea cookies beneath the chomping tread of these mechanical behemoths. Not surprising then that a substantial portion of the 220,000-acre Army Base resembles barren, scorched battlefields with ruts as deep as trenches.

But there is another side to Fort Hood, a softer and gentler side where, instead of the deafening roar of artillery fire, birdsong fills the air -- literally. A 1993 U.S. Fish and Wildlife Service Biological Opinion (BO) cites Fort Hood as containing essential nesting habitat for two endangered neotropical migratory songbirds: the golden-cheeked warbler (Dendroica chrysoparia) and the black-capped vireo (Vireo atricapillus). In conjunction with the BO, the Army manages 66,000 acres - more than 25 percent of the land on Base - for the recovery of these two endangered species. The Fort also provides a haven to wintering bald eagles, occasional visiting whooping cranes and peregrine falcons, and various other rare and endemic plant and animal species. Following the issuance of the Service's 1993 BO, Fort Hood contracted with The Nature Conservancy of Texas to research and monitor the birds. With on-Base offices in the Fort's Natural Resources Management building, The Nature Conservancy researchers have teamed up with Fish and Wildlife Service and Army biologists to gather data on the two endangered species. What they have found ? and the data they continue to collect ? constitutes the most comprehensive body of information on the birds to date.

For eons, migratory birds, mysteriously engineered by Mother Nature, have relied on their own built-in GPS systems to navigate long distances. Every March, black-capped vireos and golden-cheeked warblers - both tiny birds no larger than the palm of a child's hand - journey from their wintering grounds in Mexico and Central America to the protected habitat primarily along the east side of Fort Hood. Some birds even use the same nest sites they have occupied in previous years. Vireos prefer nesting in patchy or clumped scrubby vegetation that has a leaf cover extending to the ground; warblers build nests in mature oak-juniper woodlands, stripping the bark of Ashe junipers for building materials.

Unfortunately, both vireo and warbler populations have been in decline for decades. Black-capped vireos, which once ranged as far north as central Kansas during the breeding season, are now confined to central and west Texas and northern Mexico, with only a few tiny, scattered, remnant populations in southern Oklahoma and north Texas. Warblers only breed in the fast-disappearing habitat of central Texas. The vireo made its way onto the endangered species list in 1987; the golden-cheeked warbler followed suit in 1990.

?The major threats these birds are facing,? said Fish and Wildlife Biologist Krishna Costello, ?include habitat loss due to urban and agricultural development in both their wintering grounds and their breeding grounds. And, nest parasitism by brown-headed cowbirds.?

Brown-headed cowbirds are so named because of their association with cattle which keep grasses cropped to lengths that make it easy for them to snatch insects. Originally, the species evolved with bison and, when bison were exterminated, adapted to survival around cattle. One of the cowbird's peculiarities is that it lays eggs in the nests of other birds, which then expend parental care on cowbird young at the expense of their own young. Ultimately, the nesting success of many species of songbirds has been reduced.

?Historically, the impact of parasitism was limited,? said Fort Hood Endangered Species Program Manager John Cornelius. ?The cowbird effect on other birds were localized so long as they were associated with wandering herds of bison. But now, cowbirds are adapted to livestock. The livestock are extremely widespread across the landscape. The ecological niche for the cowbird vastly expanded with this land-use change so that cowbird numbers have increased, and they began to impact bird species across their entire range, not just locally. This has led to significant declines in a number of songbird species. Cowbirds have been documented parasitizing more than 220 species of other birds.?

Without active brown-headed cowbird population management, Fort Hood is no sanctuary from the threat of brown-headed cowbird parasitism. In fact, cowbirds are common on Base due to a 200,000-acre long-term grazing lease with the Central Texas Cattleman's Association.

?We began monitoring the black-capped vireo in 1987,? said Cornelius. ?After two years of observing parasitism above 90 percent, and extremely poor productivity, we calculated statistically that the bird would become locally extinct within ten years without immediate intervention.?

Fortunately, this has not been the case. In 1989, after intensive research on the ecology and management of brown-headed cowbirds, the team at Fort Hood began installing cowbird traps at various locations throughout the Base where cows tend to concentrate. About the size of a single- car garage, a mesh-covered trap lures cowbirds in ? with food, water, and decoys ? through a narrow slit in the top of a wood frame. Once in, cowbirds cannot get out. Females are humanely dispatched; males and the few non-target birds are released unharmed. The trapping effort yielded solid results. In 2000, studies reveal that nest parasitism has been reduced to less than 10 percent base-wide. ?Now,? said Cornelius, ?parasitism is low, productivity is high, and Fort Hood is very likely serving as a source population for vireo production.?

Currently, 33 traps on Base and 27 traps on adjoining private properties continue to thwart the cowbird threat. The successful trapping effort is also being expanded into other portions of the vireo's range.

As far as the threat of continuing habitat loss, Costello emphasized that known endangered species habitat ?must be protected.? U.S. Fish and Wildlife Service Recovery Plans for both species identify goals of attaining viable populations throughout the birds? ranges. The habitat on Fort Hood is critical to achieve the goals, for both the warbler and the vireo. Other critical areas include the Balcones Canyonlands National Wildlife Refuge, the Balcones Canyonlands Preserve managed by the City of Austin, and Government Canyon State Natural Area.

?So far,? said Costello, ?Fort Hood has followed the guidelines and requirements of the 1993 Biological Opinion and an updated 2000 BO to a tee, and in the process has produced extraordinary research and management strategies that can be applied to warbler and vireo issues range-wide. The birds are benefitting from a very good working relationship we have with the Garrison Commander and the Natural Resource staff.?

[Article by Ben Ikenson first appeared in Environmental News Network (enn.com); also appeared in People, Land & Water; and Albuquerque's Weekly Alibi]

Contact Info: Martin Valdez, 505-248-6599, martin_valdez@fws.gov
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