About the Project
Background
Gray hawks are primarily a Central American species whose northern-most range extends into southern Arizona and Texas. They were once thought to rely on riparian areas, or vegetation along waterways, and eat mostly lizards. We studied a population in the San Pedro Riparian National Conservation Area from 2011-2012 and 2014-2016 to determine how the elimination of grazing there affected their abundance, diet, and habitat use. |
Methods
In order to determine diet and productivity, we located all the nests in the study area, band and measured the chicks, and installed motion-activated cameras in a sub-set of the nests. The cameras turned on as the adults flew into the nest with food for the chicks and captured photos of the prey delivery. We later analyzed the photos to determine how much of each prey species each pair brought back. We calculated nest productivity using the number of chicks, their health, and survival rate. Paired with analysis of the area's vegetation, these data indicate what strategies this species uses and how successful they are in different habitat types.
In order to determine diet and productivity, we located all the nests in the study area, band and measured the chicks, and installed motion-activated cameras in a sub-set of the nests. The cameras turned on as the adults flew into the nest with food for the chicks and captured photos of the prey delivery. We later analyzed the photos to determine how much of each prey species each pair brought back. We calculated nest productivity using the number of chicks, their health, and survival rate. Paired with analysis of the area's vegetation, these data indicate what strategies this species uses and how successful they are in different habitat types.
Results
We found that since the elimination of grazing in the San Pedro Riparian National Conservation Area, gray hawks now nest near grasslands as well as mesquite, and are thriving on mammals as well as lizards. Prey were likely not abundant in grasslands while the area supported cattle, because livestock reduce the height and cover of the grasses that many small mammals rely on. Once cattle were removed, the grasslands regenerated and became habitat for gray hawk prey. This allowed the gray hawk population to expand into areas surrounded by grasslands. Note that gray hawks nest in cottonwood trees along the river, not the grassland or mesquite itself. These vegetation types are where they hunt, and are thus the drivers of their choice of prey. |
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FundingArizona State University's Graduate and Professional Student Council, the University of Arizona's Institute of the Environment and Graduate and Professional Student Council, Arizona Field Ornithologists, T&E Inc., the Arizona Nevada Academy of Sciences, Tucson Electrical Power, the Doris Duke Conservation Scholars Program, and contributors to a crowdfunding campaign.
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