Because birds are relatively warm objects in relation to the ambient air, a sensitive infrared sensor can detect the radiation from birds flying overhead across the sky. By pointing a thermal imaging device of 1.45° opening angle to the sky, migrating birds can be detected from 300 m up to 3000 m (Zehnder et al. 2001). Flight tracks are recorded on video and targets are classified into size classes to estimate flight altitudes. Infrared sensors work best at night under clear skies. The method detects birds at higher altitudes than ceilometer observations.
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