The radar images below show bird migration as seen in radial velocity. These Doppler measured velocities give the speed of movement toward or away from the radar. In the images below, red is toward the radar and green is away. Red and green meet at the Doppler null which identifies where birds are moving largely perpendicular to the radar and therefore have very low Doppler speeds. The direction of movement is generally perpendicular to the null.
The ground speed and direction of travel of birds depicted in velocity is the addition
of wind speed and direction and bird airspeed and heading. Wind speed and direction
therefore can exert a strong influence on the direction of migration. Echoes seen at
longer ranges from the radar are also higher since the earth and anything bound to it
pass beneath the beam at longer ranges (see this
brief description of radar imagery). Therefore birds detected at different ranges
from the radar are flying at different heights and, therefore, may be flying in different
winds. The first image depicts migration in a uniform wind field that does not change
as altitidue increases. Here, arrows showing bird direction of travel perpendicular to
the radar along the Doppler null highlight this. The second image depicts birds
migrating in conditions of strong vertical wind shear. Variation in wind direction
between low and high altitudes causes birds to become vertically stratified in their
directions of travel. Higher birds (seen at longer ranges) are moving toward the ESE
while lower birds (seen at shorter ranges) are flying north (see arrows).