Meet the Winners:

Michael T. Stewart

2024 BioOne Ambassador

BioOne Ambassador Award

An Urban Success Story? Tracking Gray Hawks in the Lower Rio Grande Valley of Texas

In a world where natural habitats are rapidly disappearing, my research focuses on the little-known Gray Hawk, a Neotropical raptor that ranges from the southwestern United States to northern Costa Rica. This species, particularly its population in Texas’s Lower Rio Grande Valley, remains shrouded in mystery, prompting my exploration into its natural history, ecology, and conservation needs. Published in the Journal of Raptor Research, this recent article addresses significant knowledge gaps for the Gray Hawk, which is state-listed as threatened in Texas. While my research is specific to a single species, it carries potential implications that extend far beyond raptors and avian species, encompassing mammals, reptiles, plants, and various other species impacted by the challenges of urbanization and habitat loss.

Through field observations and GPS tracking of both juvenile and adult hawks, this research sheds light on dispersal patterns, juvenile winter territories, and adult annual home ranges. Surprisingly, we discovered that Gray Hawks are not confined to wild areas but are utilizing unexpected urban environments within their US range. Almost half of the 54 monitored territories were located in urban or suburban settings, challenging the conventional perception of this species. This adaptability, coupled with an estimated larger state population, suggests promising conservation opportunities for Gray Hawks in Texas through raptor-friendly urban practices.

Gray Hawk with GPS Tracking

Our results showed adult Gray Hawks within our study area display year-round residence and site fidelity within relatively small annual home ranges averaging around 500 hectares. Unpaired “floater” males maintained larger home ranges and moved widely across the landscape, often visiting other hawks’ territories for a short time. These habitat types ranged from riparian corridors to suburban neighborhoods, with range size varying more by landscape type than by sex.

Juvenile Gray Hawks exhibited diverse movements after departing their natal areas, with a median dispersal date of August 11th. While most juveniles wintered in the Lower Rio Grande Valley, some embarked on longer journeys, travelling to El Salvador, Honduras, and Nicaragua. The timing of departure from the study area for the three juveniles who travelled to Central America coincided with the timing of fall migration for Gray Hawks in Arizona.

Notably, two female siblings took divergent paths, with one staying in the study area near Brownsville, Texas, and the other traveling over 1,600 km to spend the winter in El Salvador. The average date juveniles settled on winter territories was November 25th, with many of these territories in urban areas. Around March 23, 2021, juveniles resumed their exploratory movements across the study area. None of the juveniles we tracked attempted to breed in their first year.

As a state-threatened raptor that was barely studied in Texas before this research, our data significantly enhances baseline knowledge. Our findings offer valuable insights for conservation planning, from urban development policies to habitat protections on public and private lands. The adaptability showcased by this species, particularly within urban areas, highlights the need for a reevaluation of traditional conservation approaches. Gray Hawks, as a state-threatened species, could potentially play a crucial role as an umbrella species for the entire Lower Rio Grande Valley ecosystem. Focusing conservation efforts on this charismatic raptor may indirectly protect countless other species within this ecological community, fostering a more comprehensive and interconnected approach to biodiversity preservation.

Furthermore, understanding the intricacies of Gray Hawk adaptability can provide valuable insights into the delicate balance required for species to thrive in urban landscapes. This adaptability might serve as a blueprint for other wildlife struggling with similar challenges. If Gray Hawks can successfully navigate and flourish in urban areas, deciphering the underlying factors behind this success could revolutionize conservation strategies. This knowledge not only benefits the Gray Hawk but could potentially be applied to aid the adaptation and conservation of various other species impacted by the encroachment of human development. These insights offer a beacon of hope for biodiversity preservation in the face of increasing urbanization, guiding us towards more effective and tailored conservation efforts.

This response is in reference to:

Adult Home Range Size and Juvenile Movements of Gray Hawks in the Lower Rio Grande Valley, Texas, USA

Journal of Raptor Research, 57(1): 1-11. (2022). Michael T. Stewart, William S. Clark, Brian A. Millsap, Brent D. Bibles, Timothy Brush

Michael T. Stewart

Michael T. Stewart

An Urban Success Story? Tracking Gray Hawks in the Lower Rio Grande Valley of Texas

Mike Stewart has always enjoyed spending time in nature and became deeply interested in birds when a pair of Blue Jays nested outside his kitchen window.After retiring from the Army in 2019 he became a full-time student, with plans to work as an avian ecologist after completing his studies.

What drew you to the research topic you explored in your submission?

When retiring from the Army and planning to attend graduate school I had the opportunity to attend The University of Texas Rio Grande Valley. This region has many species unique to the US, or to a very limited range within the US. Many of these are understudied, and this region is also rapidly developing. I jumped at the opportunity to study a raptor with a very limited US range who has experienced the loss of over 95% of native habitat within this region. I am very interested in working to conserve species that are listed or prevent species from becoming listed.

How do you see your work contributing to public policy, citizen science, and/or science education more broadly?

My work sheds light on this population that has not been studied within Texas, the only primary literature from Texas is an observation of five chicks fledging from a nest in Bentsen State Park. This species has been state-listed as threatened since 1977 so my research can guide conservation efforts for years to come, with conservation focused on Gray Hawks having an umbrella effect for countless other species of flora and fauna. There is little research for the species as a whole, so hopefully this research will have a much broader impact for Gray Hawks throughout their range.

What are your continuing research goals for the future (near and/or far)? What topics, areas, subjects are you interested in exploring?

I am currently a PhD student at Texas A&M University Kingsville where I want to find out why Gray Hawks are selecting the areas they’re in, locate other areas that meet their needs, and determine whether or not this population is growing. I plan to do that through the use of an integrated step-selection analysis, creating a species distribution model using a maximum entropy approach, and through various models of demographic parameters. Past this degree I hope to work with birds, not necessarily raptors, in order to conserve them for future generations.

Is there anything else you want to share about you and/or your research?

I have thoroughly enjoyed the opportunity to work with such a charismatic species and I hope my work benefits Gray Hawks and inspires others. I am very grateful to the Journal of Raptor Research for nominating me for this award, it is an honor just to be nominated.


Michael T. Stewart

For information about the Raptor Research Foundation, please visit their website:

Raptor Research Foundation

For questions about BioOne or the BioOne Ambassador Award, please contact:

Amanda Rogers

BioOne Publishing
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