Geolocation by Light

Following the Migration of Lesser Kestrels of MongoliaPaul Millhouser GIS Consulting

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+

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

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

Kestrel Capture: Bal-Chatri Traprap

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

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

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

Mapping the Data: All Confidence Levels

Mapping the Data: High Confidence Only

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

AcknowledgmentsThanks to John V. Azua, Jr., Curator of Birds, and Ganchimeg Wingard,

Mongolia Program Director, Denver Zoo

Photos and data courtesy of Denver Zoo