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B. BRUDERER UND J. JOSS 



b) If a large flock with an extension larger than the puise volume is tracked, 

 the radar beam follows and extrapolâtes the position of the point of strongest 

 reflection within the flock. 



6. The variation of the cross-section with différent aspects, i.e. the echoing area 

 polar diagram was investigated. Measurements on free flying chaffinches 

 ( Fringilla coelebs ) and the common buzzard ( Buteo buteo ) showed that exact 

 calculations of track and heading are important informations for the inter- 

 prétation of absolute radar cross-sections. In good agreement with laboratory- 

 measurements of Edwards and Houghton (1959) and Houghton (1969) we 

 found largest echoing areas when the birds were broadside-on to the radar. 

 In caudal and frontal view the cross-section was reduced in the average by a 

 factor of 6 (8 dB). Fastest changes and often also the smallest echoing areas 

 occurred when the bird's body-axis was oblique with respect to the radar beam 

 (bird aspects between 25° and 70° and between 155° and 110° respectively). 



7. We see the following possibilities in applying our results to the investigation 

 of bird migration and bird hazards to aircraft: 



a) Single birds and flocks of birds may easily be distinguished by means of 

 rate and amplitude of echo fluctuations. 



b ) Many species may be recognized by examining their wing-beat pattern and 

 flight velocity as shown in a previous paper (Bruderer 1969). 



c) By means of the average cross-section the size of bird-flocks and single birds 

 may be estimated. The weight of an individual may be calculated with the 

 approximation that the average cross-section of a bird corresponds to the 

 cross-section of the water sphère with the same weight. In the same way it is 

 possible to estimate the number of individuals in a flock of known bird species. 

 The accuracy may often be improved if the cross-section of the species in 

 question is compared with that of a known individual (e.g. chaffinch) with 

 the same position of the body-axis in relation to the radar beam. 



LITERATUR 



Bruderer, B. 1969. Zur Registrierung und Interprétation von Echosignaturen an einem 



3-cm-Zielverfolgungsradar. Orn. Beob. 66: 70-88. 

 Edwards, J. and E. W. Houghton, 1959. Radar echoing area polar diagrams of birds. 



Nature, 184, 1059. 



Houghton, E. W. 1969. Radar track of two crows. R.R.E. Mémorandum No. 2483. 

 JOSS, J. 1964. Die Bestimmwtg der Riickstreuquerschnitte von Eis-Wasser-Gemischen bei 



einer Wellenlânge von 5J)5 cm, Zs.angew.Mathematik u. Physik 15: 



509-539. 



