geolocation by light: following the migration of lesser kestrels of mongolia

1. Geolocation by Light
Following the Migration of Lesser Kestrels of Mongolia
Paul Millhouser GIS Consulting

3. The Challenge
• Denver Zoo conservation biologists are studying lesser kestrels in
Mongolia and need to understand their migratory patterns
• Literature is contradictory
• Lesser kestrels are small
• Wingspan of about 2 feet
• Weight of less than 8 ounces
• Satellite tracker costs $3000+

4. The Solution
• Light-based geolocators developed by the British Antarctic Survey for
seabird research
• Determine approximate latitude and longitude
• Longitude is determined by time of sunrise and sunset
• Latitude requires estimating length of day and knowing the zenith angle at
which the unit senses sunrise and sunset
• Cost per unit of ~$200

5. The Technology
• Geolocator consists of light sensor, clock, and memory embedded in clear
epoxy.
• Simply records average light intensity for the preceding two minutes for
each minute
• Location fixes determined in post-processing

6. Kestrel Capture: Bal-Chatri Trap
rap

7. Viewing the Data
• Unit is calibrated by measuring light levels at
known location
• Light level threshold set to reflect
sunrise/sunset times and elevation angles

8. Confounding Factors
• Dense cloud cover
• Tree canopy
• Artificial lights/full moons
• Equinox +/- 2 weeks
• Length of day approximately the same at different latitudes
• Assign lower confidence levels to transitions in that period
• Poor positioning of geolocator on bird

9. Processing the Data
• View each sunrise/sunset
• Automatically filter transitions resulting in impossibly short “nights”
• Manually delete transitions that are clearly erroneous
• Assign lower confidence levels to doubtful transitions

10. Mapping the Data: All Confidence Levels

11. Mapping the Data: High Confidence Only

14. Light Geolocator vs Satellite
Light Geolocator
• Low initial cost
• No ongoing expense
• Requires recapture
• Average error +/- 185 km
• Retrospective data
Satellite
• High initial cost
• Monthly subscription
• One capture only
• Highly accurate and precise
• Real time data

15. Acknowledgments
Thanks to John V. Azua, Jr., Curator of Birds, and Ganchimeg Wingard,
Mongolia Program Director, Denver Zoo
Photos and data courtesy of Denver Zoo