2016 conservation track: geolocation by light: following the migration of lesser kestrels of...
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Geolocation by Light
Following the Migration of Lesser Kestrels of MongoliaPaul Millhouser GIS Consulting
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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+
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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
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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
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Kestrel Capture: Bal-Chatri Traprap
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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
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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
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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
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Mapping the Data: All Confidence Levels
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Mapping the Data: High Confidence Only
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Light Geolocator vs SatelliteLight 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
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AcknowledgmentsThanks to John V. Azua, Jr., Curator of Birds, and Ganchimeg Wingard,
Mongolia Program Director, Denver Zoo
Photos and data courtesy of Denver Zoo
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