. Bulletin. Natural history; Natural history. 308 Illinois Natural History Survey Bulletin Vol. 33, Art. 3 Fig. 3. - Plot of devi- ations of track directions and estimated headings, for individual thrushes, from the respective mean value of track direction and etimated heading. Datp are from Table • = SWAINSON'S THRUSH 690 O = VEERY 6901 + = CLOCKWISE - = COUNTERCLOCKWISE -30 ' -10 -40 -20 +10 ' +30 0 +20 mo +50 TRACK DIRECTION - DEVIATION FROM MEAN (degrees) counterclockwise shift into winds with a component coming from the bird's left side. Also, the airspeed estimate for T5 was greater
. Bulletin. Natural history; Natural history. 308 Illinois Natural History Survey Bulletin Vol. 33, Art. 3 Fig. 3. - Plot of devi- ations of track directions and estimated headings, for individual thrushes, from the respective mean value of track direction and etimated heading. Datp are from Table • = SWAINSON'S THRUSH 690 O = VEERY 6901 + = CLOCKWISE - = COUNTERCLOCKWISE -30 ' -10 -40 -20 +10 ' +30 0 +20 mo +50 TRACK DIRECTION - DEVIATION FROM MEAN (degrees) counterclockwise shift into winds with a component coming from the bird's left side. Also, the airspeed estimate for T5 was greater than that for T4. These changes are consistent with a compensa- tory response to lateral wind drift by a gradually altered heading and airspeed or by partial negative compensation. During the first 40 minutes of the second night's flight (Fig. 5) the track direction changed by more than 90°. The bird departed while we measured winds aloft 5 km west of the bird. The first two bearings to the bii'd were taken from this place 1 and 5 minutes after the bird took off Subsequently, seven additional bearings were taken as the tracking vehicle was driven on a circuitous route under and around the path of the bird. These bearings, and the times and places from which they were taken, are shown on the maps of Fig. 6. Although bearings were taken continually, only those above, taken carefully while the vehicle was stopped at known map points, could be used for analysis. Between 21 and 26 minutes after the bird's takeoff, while stopped, we made repeated attempts to measure the elevation angle to the bird but failed because the angle was below 30°, where ground reflections cause erratic readings. The series of bearings and the times, coupled with the winds mea- sured in the same airspace as the bird, provided data for evaluation of com- puter simulated paths of a bird's hypo- thetical constant-heading, constant-air- speed ascent through the winds aloft that we measured. The process was i
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