IRNR researchers provide new insights on endangered golden-cheeked warblers
Scientists on the Research and Management System for Endangered Species (RAMSES) team have covered a lot of ground in the past several years, conducting intensive studies in counties all over Texas for two avian species that are critical to local land management decisions.
These projects are supported by the Texas A&M Institute of Renewable Natural Resources (IRNR) and aim to determine the distribution and abundance of the golden-cheeked warbler (Setophaga chrysoparia) and the black-capped vireo (Vireo atricapilla). Both species’ breeding ranges are in central Texas.
The golden-cheeked warbler was designated as federally endangered in 1990 because of concerns about a small population size and loss and fragmentation of its woodland habitat. Since then, abundance estimates for the species have mainly relied on localized population studies on public lands and qualitative-based methods.
"Since there was a lack of data for private lands, we wanted our surveys to include private landowners, which entailed doing hours and hours of additional leg-work," said Heather Mathewson, assistant research scientist at IRNR. "Then, from 2008–2009 we sent out field crews that surveyed warbler populations all across Texas and on those cooperative landowners’ properties."
These surveys built upon work that was started on this species by the research team several years earlier. Based on that field data gathered across the breeding range, Mathewson said, the team then produced the first spatially explicit predictive model of distribution and abundance for golden-cheeked warbler. A spatially explicit model provides estimates based on features of the birds’ habitat as they change across the range of the species.
"The model provides probability of occurrence of male warblers, according to the type and size of the vegetation, and it also provides a prediction of how many birds might be there, or an estimate of abundance," she said.
"So, this model is useful to landowners, conservationists, and developers because it provides a predicted estimate of the potential loss of warblers given various changes to the landscape, such as vegetation removal. It gives them the ability to evaluate the consequences of losing warbler habitat, which is important because of the costs of mitigating for loss of an endangered species’ habitat."
The researchers recently had three peer-reviewed journal articles published stemming from the golden-cheeked warbler statewide project, adding to the growing body of publications on this species being generated by the team.
Although black-capped vireo is a neighboring species to the warbler, their differences presented the RAMSES group with challenges.
"Working with vireos is very different than working with warblers," Mathewson said. "Warblers almost always choose older oak-juniper woodlands, and it’s been previously believed that vireos usually choose the opposite—more early successional vegetation, but we’ve found that black-capped vireo habitat varies greatly across the breeding range in Texas, and that makes modeling more complex."
In 2009 and 2010, RAMSES researchers conducted a statewide survey for vireos, with the goal of developing a distribution model that can estimate the probability of vireo occurrence based on landscape and vegetative characteristics, which would be the first model of its kind.
"Because of this habitat variability, we are now conducting further surveys so that we can perfect the potential model for black-capped vireo and make it most effective for land conservation decision makers," she said.
Graduate students, primarily from the Department of Wildlife and Fisheries Sciences, form a central part of the RAMSES team. Their work covers everything from factors that affect reproduction for the two species, drought and wildfire effects on the species, and the effects of oak wilt, Mathewson said.
"We are also sincerely grateful to the landowners who allowed us access during our field work, since the majority of the surveys for both species occurred on private properties," she said.
RAMSES statewide research is supported by Texas Department of Transportation, with contributions from the U.S. Department of Defense, The Nature Conservancy, U.S. Fish and Wildlife Service, and Texas Parks and Wildlife Department.
For more information and to view the RAMSES team’s publications, visit irnr.tamu.edu/ramses.