Problems in Identifying Bird Species by Radar Signature Analyses:
Intra-Specific Variability

Emlen, S.T. 1974. In: Biological aspects of the bird-aircraft collision problem. S. Gauthreaux, ed., pp. 509-524.

The radar "signatures" of migratory birds are receiving an increasing amount of attention as scientists seek ways of identifying and evaluating unseen radar targets as potential hazards to aircraft. Numerous authors have proposed that a target can be accurately categorized, and perhaps even identified as to species, on the basis of the following parameters: cross-sectional area, wing beat frequency, length of flapping periods, length of pause periods, and air speed. But before establishing a comprehensive "dictionary" of radar signature types, it is important to measure the intra-specific variability of these parameters.

During the spring of 1972, I performed a series of orientation experiments using the FPS-16 tracking radar at NASA - Wallops Island. Individual white-throated sparrows were taken aloft at night and re-released at migratory altitudes. The tracking radar then plotted the bird's course as it selected its direction and continued its migratory flight. AGC records were obtained from over 50 sparrows flying in a variety of wind and cloud conditions. Preliminary analysis of these AGC's indicates the following: all of the signature parameters listed above (excluding cross-sectional area) proved to be variable. The wing beat frequency exhibited the smallest variance, but close examination suggests a slight trend of increasing flap rate with increasing air speed. The lengths of the "flap" and "pause" periods were highly variable and I question their diagnostic value in signature identifications. Air speed shoed an inverse trend with wind force, the latter measured along the heading vector. In other words, the sparrows appear to adjust their air speed to the winds aloft in such a manner as to minimize changes in ground speed.

These results should serve as a warning against over-zealous categorization of radar targets by signature analysis. But they also show the potential of how studies of the variability of signature can lead to a better understanding of the aerodynamic strategies employed by migrants aloft.